diff --git a/docs/resources/authorization_folder_role_assignment.md b/docs/resources/authorization_folder_role_assignment.md
index 9b5707efd..a1495e865 100644
--- a/docs/resources/authorization_folder_role_assignment.md
+++ b/docs/resources/authorization_folder_role_assignment.md
@@ -43,7 +43,7 @@ import {
 
 - `resource_id` (String) folder Resource to assign the role to.
 - `role` (String) Role to be assigned. Available roles can be queried using stackit-cli: `stackit curl https://authorization.api.stackit.cloud/v2/permissions`
-- `subject` (String) Identifier of user, service account or client. Usually email address or name in case of clients
+- `subject` (String) Identifier of user, service account or client. Usually email address or name in case of clients. All letters must be lowercased.
 
 ### Read-Only
 
diff --git a/docs/resources/authorization_organization_role_assignment.md b/docs/resources/authorization_organization_role_assignment.md
index cf965f7ab..46174f1cc 100644
--- a/docs/resources/authorization_organization_role_assignment.md
+++ b/docs/resources/authorization_organization_role_assignment.md
@@ -36,7 +36,7 @@ import {
 
 - `resource_id` (String) organization Resource to assign the role to.
 - `role` (String) Role to be assigned. Available roles can be queried using stackit-cli: `stackit curl https://authorization.api.stackit.cloud/v2/permissions`
-- `subject` (String) Identifier of user, service account or client. Usually email address or name in case of clients
+- `subject` (String) Identifier of user, service account or client. Usually email address or name in case of clients. All letters must be lowercased.
 
 ### Read-Only
 
diff --git a/docs/resources/authorization_project_role_assignment.md b/docs/resources/authorization_project_role_assignment.md
index 9317957ed..6e7bd0c1c 100644
--- a/docs/resources/authorization_project_role_assignment.md
+++ b/docs/resources/authorization_project_role_assignment.md
@@ -43,7 +43,7 @@ import {
 
 - `resource_id` (String) project Resource to assign the role to.
 - `role` (String) Role to be assigned. Available roles can be queried using stackit-cli: `stackit curl https://authorization.api.stackit.cloud/v2/permissions`
-- `subject` (String) Identifier of user, service account or client. Usually email address or name in case of clients
+- `subject` (String) Identifier of user, service account or client. Usually email address or name in case of clients. All letters must be lowercased.
 
 ### Read-Only
 
diff --git a/stackit/internal/services/authorization/roleassignments/resource.go b/stackit/internal/services/authorization/roleassignments/resource.go
index 969da3cde..0925d17c9 100644
--- a/stackit/internal/services/authorization/roleassignments/resource.go
+++ b/stackit/internal/services/authorization/roleassignments/resource.go
@@ -99,7 +99,7 @@ func (r *roleAssignmentResource) Schema(_ context.Context, _ resource.SchemaRequ
 		"id":          "Terraform's internal resource identifier. It is structured as \"`resource_id`,`role`,`subject`\".",
 		"resource_id": fmt.Sprintf("%s Resource to assign the role to.", r.apiName),
 		"role":        "Role to be assigned. Available roles can be queried using stackit-cli: `stackit curl https://authorization.api.stackit.cloud/v2/permissions`",
-		"subject":     "Identifier of user, service account or client. Usually email address or name in case of clients",
+		"subject":     "Identifier of user, service account or client. Usually email address or name in case of clients. All letters must be lowercased.",
 	}
 
 	resp.Schema = schema.Schema{
@@ -133,6 +133,9 @@ func (r *roleAssignmentResource) Schema(_ context.Context, _ resource.SchemaRequ
 			"subject": schema.StringAttribute{
 				Description: descriptions["subject"],
 				Required:    true,
+				Validators: []validator.String{
+					validate.IsLowercased(),
+				},
 				PlanModifiers: []planmodifier.String{
 					stringplanmodifier.RequiresReplace(),
 				},
diff --git a/stackit/internal/validate/validate.go b/stackit/internal/validate/validate.go
index 0af0f3c64..929285d9d 100644
--- a/stackit/internal/validate/validate.go
+++ b/stackit/internal/validate/validate.go
@@ -369,3 +369,27 @@ func ValidNoTrailingNewline() *Validator {
 		},
 	}
 }
+
+// IsLowercased returns a Validator that checks if all letters in the input string are lowercase.
+// Examples:
+// - "example": valid
+// - "example123": valid, numbers are not affected by case
+// - "exaMPle123": invalid
+func IsLowercased() *Validator {
+	description := `All letters in the value must be lowercase as defined by gos unicode.IsLower(rune).`
+	return &Validator{
+		description: description,
+		validate: func(_ context.Context, request validator.StringRequest, response *validator.StringResponse) {
+			val := request.ConfigValue.ValueString()
+			lowercased := strings.ToLower(val)
+			if val != lowercased {
+				response.Diagnostics.Append(validatordiag.InvalidAttributeValueDiagnostic(
+					request.Path,
+					description,
+					val,
+				))
+				return
+			}
+		},
+	}
+}
diff --git a/stackit/internal/validate/validate_test.go b/stackit/internal/validate/validate_test.go
index 3436a7a1f..c08a70d14 100644
--- a/stackit/internal/validate/validate_test.go
+++ b/stackit/internal/validate/validate_test.go
@@ -966,3 +966,45 @@ func TestValidNoTrailingNewline(t *testing.T) {
 		})
 	}
 }
+
+func TestIsLowercased(t *testing.T) {
+	tests := []struct {
+		description string
+		input       string
+		wantErr     bool
+	}{
+		{
+			description: "empty",
+			input:       "",
+		},
+		{
+			description: "simple lowercase",
+			input:       "lowercase",
+		},
+		{
+			description: "lowercase with numbers and symbols",
+			input:       "lowercase123!@#",
+		},
+		{
+			description: "uppercase letters",
+			input:       "UpperCase",
+			wantErr:     true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.description, func(t *testing.T) {
+			r := validator.StringResponse{}
+			IsLowercased().ValidateString(context.Background(), validator.StringRequest{
+				ConfigValue: types.StringValue(tt.input),
+			}, &r)
+
+			if tt.wantErr && !r.Diagnostics.HasError() {
+				t.Fatalf("Expected validation to fail for input: %q", tt.input)
+			}
+			if !tt.wantErr && r.Diagnostics.HasError() {
+				t.Fatalf("Expected validation to succeed for input: %q, but got errors: %v", tt.input, r.Diagnostics.Errors())
+			}
+		})
+	}
+}