diff --git a/.changelog/38804.txt b/.changelog/38804.txt
new file mode 100644
index 00000000000..2417486ac49
--- /dev/null
+++ b/.changelog/38804.txt
@@ -0,0 +1,3 @@
+```release-note:bug
+resource/aws_ecs_task_definition: Prevent lowercasing of the first character of JSON keys in `container_definitions.dockerLabels`
+```
\ No newline at end of file
diff --git a/internal/json/transform.go b/internal/json/transform.go
deleted file mode 100644
index 34ffbe8d551..00000000000
--- a/internal/json/transform.go
+++ /dev/null
@@ -1,51 +0,0 @@
-// Copyright (c) HashiCorp, Inc.
-// SPDX-License-Identifier: MPL-2.0
-
-package json
-
-import (
-	"unicode"
-	"unicode/utf8"
-
-	"github.com/hashicorp/terraform-provider-aws/internal/json/ujson"
-)
-
-// KeyFirstLower converts the first letter of each key to lowercase.
-func KeyFirstLower(in []byte) []byte {
-	out := make([]byte, 0, len(in))
-
-	err := ujson.Walk(in, func(_ int, key, value []byte) bool {
-		// Write to output.
-		if len(out) != 0 && ujson.ShouldAddComma(value, out[len(out)-1]) {
-			out = append(out, ',')
-		}
-		// key is the raw key of the current object or empty otherwise.
-		// It can be a double-quoted string or empty.
-		switch len(key) {
-		case 0:
-		case 1, 2:
-			// Empty key.
-			out = append(out, key...)
-		default:
-		}
-		if len(key) > 0 {
-			out = append(out, '"')
-			r, n := utf8.DecodeRune(key[1:])
-			r = unicode.ToLower(r)
-			low := make([]byte, utf8.RuneLen(r))
-			utf8.EncodeRune(low, r)
-			out = append(out, low...)
-			out = append(out, key[n+1:]...)
-			out = append(out, ':')
-		}
-		out = append(out, value...)
-
-		return true
-	})
-
-	if err != nil {
-		return nil
-	}
-
-	return out
-}
diff --git a/internal/json/transform_test.go b/internal/json/transform_test.go
deleted file mode 100644
index e9b311f7f9e..00000000000
--- a/internal/json/transform_test.go
+++ /dev/null
@@ -1,68 +0,0 @@
-// Copyright (c) HashiCorp, Inc.
-// SPDX-License-Identifier: MPL-2.0
-
-package json_test
-
-import (
-	"testing"
-
-	"github.com/hashicorp/terraform-provider-aws/internal/json"
-)
-
-func TestKeyFirstLower(t *testing.T) {
-	t.Parallel()
-
-	testCases := []struct {
-		testName string
-		input    string
-		want     string
-	}{
-		{
-			testName: "empty JSON",
-			input:    "{}",
-			want:     "{}",
-		},
-		{
-			testName: "single field, lowercase",
-			input:    `{ "key": 42 }`,
-			want:     `{"key":42}`,
-		},
-		{
-			testName: "single field, uppercase",
-			input:    `{ "Key": 42 }`,
-			want:     `{"key":42}`,
-		},
-		{
-			testName: "multiple fields",
-			input: `
-[
-  {
-    "Name": "FIRST",
-    "Image": "alpine",
-    "Cpu": 10,
-    "Memory": 512,
-    "Essential": true,
-    "PortMappings": [
-      {
-        "ContainerPort": 80,
-        "HostPort": 80
-      }
-    ]
-  }
-]
-			`,
-			want: `[{"name":"FIRST","image":"alpine","cpu":10,"memory":512,"essential":true,"portMappings":[{"containerPort":80,"hostPort":80}]}]`,
-		},
-	}
-
-	for _, testCase := range testCases {
-		testCase := testCase
-		t.Run(testCase.testName, func(t *testing.T) {
-			t.Parallel()
-
-			if got, want := string(json.KeyFirstLower([]byte(testCase.input))), testCase.want; got != want {
-				t.Errorf("KeyFirstLower(%q) = %q, want %q", testCase.input, got, want)
-			}
-		})
-	}
-}
diff --git a/internal/service/ecs/task_definition_equivalency.go b/internal/service/ecs/container_definitions.go
similarity index 71%
rename from internal/service/ecs/task_definition_equivalency.go
rename to internal/service/ecs/container_definitions.go
index 6947d99a884..1dbcbd2bc5d 100644
--- a/internal/service/ecs/task_definition_equivalency.go
+++ b/internal/service/ecs/container_definitions.go
@@ -4,11 +4,16 @@
 package ecs
 
 import (
+	"fmt"
 	"sort"
+	_ "unsafe" // Required for go:linkname
 
 	"github.com/aws/aws-sdk-go-v2/aws"
+	_ "github.com/aws/aws-sdk-go-v2/service/ecs" // Required for go:linkname
 	awstypes "github.com/aws/aws-sdk-go-v2/service/ecs/types"
+	smithyjson "github.com/aws/smithy-go/encoding/json"
 	tfjson "github.com/hashicorp/terraform-provider-aws/internal/json"
+	itypes "github.com/hashicorp/terraform-provider-aws/internal/types"
 )
 
 func containerDefinitionsAreEquivalent(def1, def2 string, isAWSVPC bool) (bool, error) {
@@ -143,3 +148,36 @@ func (cd containerDefinitions) orderContainers() {
 		return aws.ToString(cd[i].Name) < aws.ToString(cd[j].Name)
 	})
 }
+
+// Dirty hack to avoid any backwards compatibility issues with the AWS SDK for Go v2 migration.
+// Reach down into the SDK and use the same serialization function that the SDK uses.
+//
+//go:linkname serializeContainerDefinitions github.com/aws/aws-sdk-go-v2/service/ecs.awsAwsjson11_serializeDocumentContainerDefinitions
+func serializeContainerDefinitions(v []awstypes.ContainerDefinition, value smithyjson.Value) error
+
+func flattenContainerDefinitions(apiObjects []awstypes.ContainerDefinition) (string, error) {
+	jsonEncoder := smithyjson.NewEncoder()
+	err := serializeContainerDefinitions(apiObjects, jsonEncoder.Value)
+
+	if err != nil {
+		return "", err
+	}
+
+	return jsonEncoder.String(), nil
+}
+
+func expandContainerDefinitions(tfString string) ([]awstypes.ContainerDefinition, error) {
+	var apiObjects []awstypes.ContainerDefinition
+
+	if err := tfjson.DecodeFromString(tfString, &apiObjects); err != nil {
+		return nil, err
+	}
+
+	for i, apiObject := range apiObjects {
+		if itypes.IsZero(&apiObject) {
+			return nil, fmt.Errorf("invalid container definition supplied at index (%d)", i)
+		}
+	}
+
+	return apiObjects, nil
+}
diff --git a/internal/service/ecs/task_definition_equivalency_test.go b/internal/service/ecs/container_definitions_test.go
similarity index 100%
rename from internal/service/ecs/task_definition_equivalency_test.go
rename to internal/service/ecs/container_definitions_test.go
diff --git a/internal/service/ecs/task_definition.go b/internal/service/ecs/task_definition.go
index b087c41fa85..c4e8a50be4c 100644
--- a/internal/service/ecs/task_definition.go
+++ b/internal/service/ecs/task_definition.go
@@ -26,10 +26,8 @@ import (
 	"github.com/hashicorp/terraform-provider-aws/internal/errs"
 	"github.com/hashicorp/terraform-provider-aws/internal/errs/sdkdiag"
 	"github.com/hashicorp/terraform-provider-aws/internal/flex"
-	tfjson "github.com/hashicorp/terraform-provider-aws/internal/json"
 	tftags "github.com/hashicorp/terraform-provider-aws/internal/tags"
 	"github.com/hashicorp/terraform-provider-aws/internal/tfresource"
-	itypes "github.com/hashicorp/terraform-provider-aws/internal/types"
 	"github.com/hashicorp/terraform-provider-aws/internal/verify"
 	"github.com/hashicorp/terraform-provider-aws/names"
 )
@@ -1211,35 +1209,6 @@ func flattenFSxWinVolumeAuthorizationConfig(config *awstypes.FSxWindowsFileServe
 	return items
 }
 
-func flattenContainerDefinitions(apiObjects []awstypes.ContainerDefinition) (string, error) {
-	json, err := tfjson.EncodeToBytes(apiObjects)
-	if err != nil {
-		return "", err
-	}
-
-	// Remove empty fields and convert first character of keys to lowercase.
-	json = tfjson.RemoveEmptyFields(json)
-	json = tfjson.KeyFirstLower(json)
-
-	return string(json), nil
-}
-
-func expandContainerDefinitions(tfString string) ([]awstypes.ContainerDefinition, error) {
-	var apiObjects []awstypes.ContainerDefinition
-
-	if err := tfjson.DecodeFromString(tfString, &apiObjects); err != nil {
-		return nil, err
-	}
-
-	for i, apiObject := range apiObjects {
-		if itypes.IsZero(&apiObject) {
-			return nil, fmt.Errorf("invalid container definition supplied at index (%d)", i)
-		}
-	}
-
-	return apiObjects, nil
-}
-
 func expandTaskDefinitionEphemeralStorage(config []interface{}) *awstypes.EphemeralStorage {
 	configMap := config[0].(map[string]interface{})
 
diff --git a/internal/service/ecs/task_definition_test.go b/internal/service/ecs/task_definition_test.go
index d4b78e2b316..a8351554c32 100644
--- a/internal/service/ecs/task_definition_test.go
+++ b/internal/service/ecs/task_definition_test.go
@@ -1337,6 +1337,133 @@ func TestAccECSTaskDefinition_v5590ContainerDefinitionsRegression(t *testing.T)
 	})
 }
 
+// https://github.com/hashicorp/terraform-provider-aws/issues/38779.
+func TestAccECSTaskDefinition_containerDefinitionEmptyPortMappings(t *testing.T) {
+	ctx := acctest.Context(t)
+	var def awstypes.TaskDefinition
+	rName := sdkacctest.RandomWithPrefix(acctest.ResourcePrefix)
+	resourceName := "aws_ecs_task_definition.test"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { acctest.PreCheck(ctx, t) },
+		ErrorCheck:   acctest.ErrorCheck(t, names.ECSServiceID),
+		CheckDestroy: testAccCheckTaskDefinitionDestroy(ctx),
+		Steps: []resource.TestStep{
+			{
+				ExternalProviders: map[string]resource.ExternalProvider{
+					"aws": {
+						Source:            "hashicorp/aws",
+						VersionConstraint: "5.58.0",
+					},
+				},
+				Config: testAccTaskDefinitionConfig_containerDefinitionEmptyPortMappings(rName, "alpine"),
+				Check: resource.ComposeAggregateTestCheckFunc(
+					testAccCheckTaskDefinitionExists(ctx, resourceName, &def),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "length(@)", acctest.Ct1),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].cpu", acctest.Ct10),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].essential", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].image", "alpine"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].memory", "512"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].name", "first"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "length([0].portMappings)", acctest.Ct0),
+				),
+			},
+			// At v5.59.0 and v5.60.0, JSON keys were returned with leading capital letters.
+			{
+				ExternalProviders: map[string]resource.ExternalProvider{
+					"aws": {
+						Source:            "hashicorp/aws",
+						VersionConstraint: "5.60.0",
+					},
+				},
+				Config: testAccTaskDefinitionConfig_containerDefinitionEmptyPortMappings(rName, "jenkins"),
+				Check: resource.ComposeAggregateTestCheckFunc(
+					testAccCheckTaskDefinitionExists(ctx, resourceName, &def),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "length(@)", acctest.Ct1),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].Cpu", acctest.Ct10),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].Essential", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].Image", "jenkins"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].Memory", "512"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].Name", "first"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "length([0].PortMappings)", acctest.Ct0),
+				),
+			},
+			// At v5.61.0, all empty values were removed.
+			{
+				ExternalProviders: map[string]resource.ExternalProvider{
+					"aws": {
+						Source:            "hashicorp/aws",
+						VersionConstraint: "5.61.0",
+					},
+				},
+				Config: testAccTaskDefinitionConfig_containerDefinitionEmptyPortMappings(rName, "nginx"),
+				Check: resource.ComposeAggregateTestCheckFunc(
+					testAccCheckTaskDefinitionExists(ctx, resourceName, &def),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "length(@)", acctest.Ct1),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].cpu", acctest.Ct10),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].essential", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].image", "nginx"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].memory", "512"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].name", "first"),
+					acctest.CheckResourceAttrJMESNotExists(resourceName, "container_definitions", "[0].portMappings"),
+				),
+			},
+			// At v5.62.0, fidelity with v5.58 was restored.
+			{
+				ProtoV5ProviderFactories: acctest.ProtoV5ProviderFactories,
+				Config:                   testAccTaskDefinitionConfig_containerDefinitionEmptyPortMappings(rName, "alpine"),
+				Check: resource.ComposeAggregateTestCheckFunc(
+					testAccCheckTaskDefinitionExists(ctx, resourceName, &def),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "length(@)", acctest.Ct1),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].cpu", acctest.Ct10),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].essential", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].image", "alpine"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].memory", "512"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].name", "first"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "length([0].portMappings)", acctest.Ct0),
+				),
+			},
+		},
+	})
+}
+
+// https://github.com/hashicorp/terraform-provider-aws/issues/38782.
+func TestAccECSTaskDefinition_containerDefinitionDockerLabels(t *testing.T) {
+	ctx := acctest.Context(t)
+	var def awstypes.TaskDefinition
+	rName := sdkacctest.RandomWithPrefix(acctest.ResourcePrefix)
+	resourceName := "aws_ecs_task_definition.test"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:                 func() { acctest.PreCheck(ctx, t) },
+		ErrorCheck:               acctest.ErrorCheck(t, names.ECSServiceID),
+		ProtoV5ProviderFactories: acctest.ProtoV5ProviderFactories,
+		CheckDestroy:             testAccCheckTaskDefinitionDestroy(ctx),
+		Steps: []resource.TestStep{
+			{
+				Config: testAccTaskDefinitionConfig_containerDefinitionDockerLabels(rName, "alpine"),
+				Check: resource.ComposeAggregateTestCheckFunc(
+					testAccCheckTaskDefinitionExists(ctx, resourceName, &def),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "length(@)", acctest.Ct1),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "keys([0].dockerLabels) | contains(@, 'PROMETHEUS_EXPORTER_JOB_NAME')", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].dockerLabels.PROMETHEUS_EXPORTER_JOB_NAME", "my-job"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "keys([0].dockerLabels) | contains(@, 'PROMETHEUS_EXPORTER_PATH')", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].dockerLabels.PROMETHEUS_EXPORTER_PATH", "/prometheus"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "keys([0].dockerLabels) | contains(@, 'PROMETHEUS_EXPORTER_PORT')", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].dockerLabels.PROMETHEUS_EXPORTER_PORT", "12345"),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "keys([0].dockerLabels) | contains(@, 'PROMETHEUS_TARGET')", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].dockerLabels.PROMETHEUS_TARGET", acctest.CtTrue),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "keys([0].dockerLabels) | contains(@, 'pROMETHEUS_EXPORTER_JOB_NAME')", acctest.CtFalse),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "keys([0].dockerLabels) | contains(@, 'pROMETHEUS_EXPORTER_PATH')", acctest.CtFalse),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "keys([0].dockerLabels) | contains(@, 'pROMETHEUS_EXPORTER_PORT')", acctest.CtFalse),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "keys([0].dockerLabels) | contains(@, 'pROMETHEUS_TARGET')", acctest.CtFalse),
+					acctest.CheckResourceAttrJMES(resourceName, "container_definitions", "[0].image", "alpine"),
+				),
+			},
+		},
+	})
+}
+
 func testAccCheckTaskDefinitionProxyConfiguration(after *awstypes.TaskDefinition, containerName string, proxyType string,
 	ignoredUid string, ignoredGid string, appPorts string, proxyIngressPort string, proxyEgressPort string,
 	egressIgnoredPorts string, egressIgnoredIPs string) resource.TestCheckFunc {
@@ -3261,3 +3388,52 @@ resource "aws_ecs_task_definition" "test" {
 }
 `, rName, image)
 }
+
+func testAccTaskDefinitionConfig_containerDefinitionEmptyPortMappings(rName, image string) string {
+	return fmt.Sprintf(`
+resource "aws_ecs_task_definition" "test" {
+  family = %[1]q
+  container_definitions = jsonencode([
+    {
+      name         = "first"
+      image        = %[2]q
+      cpu          = 10
+      memory       = 512
+      essential    = true
+      portMappings = []
+    }
+  ])
+}
+`, rName, image)
+}
+
+func testAccTaskDefinitionConfig_containerDefinitionDockerLabels(rName, image string) string {
+	return fmt.Sprintf(`
+resource "aws_ecs_task_definition" "test" {
+  family = %[1]q
+  container_definitions = jsonencode([
+    {
+      name      = "first"
+      image     = %[2]q
+      cpu       = 10
+      memory    = 512
+      essential = true
+
+      portMappings = [
+        {
+          containerPort = 80
+          hostPort      = 80
+        }
+      ]
+
+      dockerLabels = {
+        "PROMETHEUS_TARGET"            = "true"
+        "PROMETHEUS_EXPORTER_PORT"     = "12345"
+        "PROMETHEUS_EXPORTER_PATH"     = "/prometheus"
+        "PROMETHEUS_EXPORTER_JOB_NAME" = "my-job"
+      }
+    }
+  ])
+}
+`, rName, image)
+}