diff --git a/internal/services/appservice/helpers/fx_strings.go b/internal/services/appservice/helpers/fx_strings.go
index dbb9498ab532..02468f12e540 100644
--- a/internal/services/appservice/helpers/fx_strings.go
+++ b/internal/services/appservice/helpers/fx_strings.go
@@ -63,6 +63,9 @@ func decodeApplicationStackLinux(fxString string) ApplicationStackLinux {
 		if strings.HasPrefix(javaParts[0], "17") {
 			result.JavaVersion = "17"
 		}
+		if strings.HasPrefix(javaParts[0], "21") {
+			result.JavaVersion = "21"
+		}
 		result.JavaServerVersion = javaParts[0]
 
 	case FxStringPrefixTomcat:
@@ -278,6 +281,22 @@ func JavaLinuxFxStringBuilder(javaMajorVersion, javaServer, javaServerVersion st
 		default:
 			return pointer.To(fmt.Sprintf("%s|%s-java17", javaServer, javaServerVersion)), nil
 		}
+	case "21":
+		switch javaServer {
+		case LinuxJavaServerJava:
+			if len(strings.Split(javaServerVersion, ".")) == 3 {
+				return pointer.To(fmt.Sprintf("%s|%s", LinuxJavaServerJava, javaServerVersion)), nil // "JAVA|21.0.4"
+			} else {
+				return pointer.To(fmt.Sprintf("%s|%s-java21", LinuxJavaServerJava, javaServerVersion)), nil // "JAVA|21-21"
+			}
+
+		case LinuxJavaServerTomcat:
+			return pointer.To(fmt.Sprintf("%s|%s-java21", LinuxJavaServerTomcat, javaServerVersion)), nil // e,g, TOMCAT|10.0-java21 / TOMCAT|10.0.20-java21
+		case LinuxJavaServerJboss:
+			return nil, fmt.Errorf("java 21 is not supported on %s", LinuxJavaServerJboss)
+		default:
+			return pointer.To(fmt.Sprintf("%s|%s-java21", javaServer, javaServerVersion)), nil
+		}
 
 	default:
 		return pointer.To(fmt.Sprintf("%s|%s-%s", javaServer, javaServerVersion, javaMajorVersion)), nil
diff --git a/internal/services/appservice/linux_web_app_resource_test.go b/internal/services/appservice/linux_web_app_resource_test.go
index 70e7cc54004c..c93fb0edd654 100644
--- a/internal/services/appservice/linux_web_app_resource_test.go
+++ b/internal/services/appservice/linux_web_app_resource_test.go
@@ -1225,7 +1225,7 @@ func TestAccLinuxWebApp_withJre21Java(t *testing.T) {
 			Config: r.java(data, "21", "JAVA", "21"),
 			Check: acceptance.ComposeTestCheckFunc(
 				check.That(data.ResourceName).ExistsInAzure(r),
-				check.That(data.ResourceName).Key("site_config.0.linux_fx_version").HasValue("JAVA|21-21"),
+				check.That(data.ResourceName).Key("site_config.0.linux_fx_version").HasValue("JAVA|21-java21"),
 			),
 		},
 		data.ImportStep("site_credential.0.password"),