diff --git a/github/resource_github_team_members.go b/github/resource_github_team_members.go
index e56bf0ac38..c11aff9a89 100644
--- a/github/resource_github_team_members.go
+++ b/github/resource_github_team_members.go
@@ -63,13 +63,21 @@ func resourceGithubTeamMembers() *schema.Resource {
 				Type:        schema.TypeSet,
 				Required:    true,
 				Description: "List of users that should be members of the team.",
+				Set: func(v any) int {
+					username, _ := v.(map[string]any)["username"].(string)
+					return schema.HashString(strings.ToLower(username))
+				},
 				Elem: &schema.Resource{
 					Schema: map[string]*schema.Schema{
 						"username": {
 							Type:             schema.TypeString,
 							Required:         true,
 							DiffSuppressFunc: caseInsensitive(),
-							Description:      "User to add to the team.",
+							StateFunc: func(v any) string {
+								val, _ := v.(string)
+								return strings.ToLower(val)
+							},
+							Description: "User to add to the team.",
 						},
 						"role": {
 							Type:             schema.TypeString,
diff --git a/github/resource_github_team_members_test.go b/github/resource_github_team_members_test.go
index fcb6e10253..6cc011f7c7 100644
--- a/github/resource_github_team_members_test.go
+++ b/github/resource_github_team_members_test.go
@@ -3,8 +3,10 @@ package github
 import (
 	"fmt"
 	"strconv"
+	"strings"
 	"testing"
 
+	"github.com/hashicorp/terraform-plugin-testing/compare"
 	"github.com/hashicorp/terraform-plugin-testing/helper/resource"
 	"github.com/hashicorp/terraform-plugin-testing/knownvalue"
 	"github.com/hashicorp/terraform-plugin-testing/plancheck"
@@ -18,6 +20,7 @@ func TestAccGithubTeamMembers(t *testing.T) {
 
 	skipUnlessHasOrgs(t)
 	skipUnlessHasOrgUser1(t)
+	flippedCaseUsername := flipUsernameCase(testAccConf.testOrgUser1)
 
 	t.Run("team_by_slug", func(t *testing.T) {
 		t.Parallel()
@@ -33,7 +36,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetSlug(), testAccConf.testOrgUser1)
+`, team.GetSlug(), flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -71,7 +74,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetSlug(), testAccConf.testOrgUser1)
+`, team.GetSlug(), flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -110,7 +113,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetID(), testAccConf.testOrgUser1)
+`, team.GetID(), flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -148,7 +151,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetSlug(), testAccConf.testOrgUser1)
+`, team.GetSlug(), flippedCaseUsername)
 
 		configMigrate := fmt.Sprintf(`
 resource "github_team_members" "test" {
@@ -159,7 +162,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetSlug(), testAccConf.testOrgUser1)
+`, team.GetSlug(), flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -201,7 +204,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetID(), testAccConf.testOrgUser1)
+`, team.GetID(), flippedCaseUsername)
 
 		configMigrate := fmt.Sprintf(`
 resource "github_team_members" "test" {
@@ -212,7 +215,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetSlug(), testAccConf.testOrgUser1)
+`, team.GetSlug(), flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -257,7 +260,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetSlug(), testAccConf.testOrgUser1)
+`, team.GetSlug(), flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -296,7 +299,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetSlug(), testAccConf.testOrgUser1)
+`, team.GetSlug(), flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -326,7 +329,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, testAccConf.testOrgUser1)
+`, flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -364,7 +367,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, team.GetSlug(), testAccConf.testOrgUser1)
+`, team.GetSlug(), flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -403,7 +406,7 @@ resource "github_team_members" "test" {
     role     = "maintainer"
   }
 }
-`, testAccConf.testOrgUser1)
+`, flippedCaseUsername)
 
 		resource.Test(t, resource.TestCase{
 			ProviderFactories: providerFactories,
@@ -430,4 +433,49 @@ resource "github_team_members" "test" {
 			},
 		})
 	})
+
+	t.Run("is_case_insensitive", func(t *testing.T) {
+		t.Parallel()
+
+		team := mustCreateTestTeam(t, nil)
+
+		config := fmt.Sprintf(`
+resource "github_team_members" "test" {
+  team_slug = "%s"
+
+  members {
+    username = "%%s"
+    role     = "maintainer"
+  }
+}
+`, team.GetSlug())
+
+		usernameIsSameComparer := statecheck.CompareValue(compare.ValuesSame())
+		resource.Test(t, resource.TestCase{
+			PreCheck:          func() { skipUnlessHasOrgs(t) },
+			ProviderFactories: providerFactories,
+			Steps: []resource.TestStep{
+				{
+					Config: fmt.Sprintf(config, flippedCaseUsername),
+					ConfigStateChecks: []statecheck.StateCheck{
+						usernameIsSameComparer.AddStateValue("github_team_members.test", tfjsonpath.New("members").AtSliceIndex(0).AtMapKey("username")),
+						statecheck.ExpectKnownValue("github_team_members.test", tfjsonpath.New("members"), knownvalue.SetSizeExact(1)),
+						statecheck.ExpectKnownValue("github_team_members.test", tfjsonpath.New("members").AtSliceIndex(0).AtMapKey("username"), knownvalue.StringExact(strings.ToLower(testAccConf.testOrgUser1))),
+					},
+				},
+				{
+					Config: fmt.Sprintf(config, testAccConf.testOrgUser1),
+					ConfigPlanChecks: resource.ConfigPlanChecks{
+						PreApply: []plancheck.PlanCheck{
+							plancheck.ExpectResourceAction("github_team_members.test", plancheck.ResourceActionNoop),
+						},
+					},
+					ConfigStateChecks: []statecheck.StateCheck{
+						usernameIsSameComparer.AddStateValue("github_team_members.test", tfjsonpath.New("members").AtSliceIndex(0).AtMapKey("username")),
+						statecheck.ExpectKnownValue("github_team_members.test", tfjsonpath.New("members"), knownvalue.SetSizeExact(1)),
+					},
+				},
+			},
+		})
+	})
 }