diff --git a/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/OAuth2TokenIntrospection.java b/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/OAuth2TokenIntrospection.java
index 660604e43..58f0629d7 100644
--- a/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/OAuth2TokenIntrospection.java
+++ b/oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/OAuth2TokenIntrospection.java
@@ -305,7 +305,7 @@ private void validate() {
 						"aud must be of type List");
 			}
 			if (this.claims.containsKey(OAuth2TokenIntrospectionClaimNames.ISS)) {
-				validateURL(this.claims.get(OAuth2TokenIntrospectionClaimNames.ISS), "iss must be a valid URL");
+				validateIssuer(this.claims.get(OAuth2TokenIntrospectionClaimNames.ISS), "iss must not be empty");
 			}
 		}
 
@@ -326,16 +326,10 @@ private void acceptClaimValues(String name, Consumer<List<String>> valuesConsume
 			valuesConsumer.accept(values);
 		}
 
-		private static void validateURL(Object url, String errorMessage) {
-			if (URL.class.isAssignableFrom(url.getClass())) {
-				return;
-			}
-
-			try {
-				new URI(url.toString()).toURL();
-			}
-			catch (Exception ex) {
-				throw new IllegalArgumentException(errorMessage, ex);
+		private static void validateIssuer(Object url, String errorMessage) {
+			String str = url.toString();
+			if (str.isBlank()) {
+				throw new IllegalArgumentException(errorMessage);
 			}
 		}
 
diff --git a/oauth2-authorization-server/src/test/java/org/springframework/security/oauth2/server/authorization/OAuth2TokenIntrospectionTests.java b/oauth2-authorization-server/src/test/java/org/springframework/security/oauth2/server/authorization/OAuth2TokenIntrospectionTests.java
new file mode 100644
index 000000000..ee6478e48
--- /dev/null
+++ b/oauth2-authorization-server/src/test/java/org/springframework/security/oauth2/server/authorization/OAuth2TokenIntrospectionTests.java
@@ -0,0 +1,73 @@
+package org.springframework.security.oauth2.server.authorization;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.security.oauth2.core.OAuth2TokenIntrospectionClaimNames;
+
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class OAuth2TokenIntrospectionTests {
+
+	@Test
+	void buildWhenIssuerIsNonUriStringThenDoesNotThrow() {
+		String issuer = "client-id-123"; // plain string, not a URI
+
+		assertThatCode(() -> {
+			OAuth2TokenIntrospection token =
+					OAuth2TokenIntrospection.builder(true)
+							.issuer(issuer)
+							.subject("user-123")
+							.build();
+
+			Object issClaim = token.getClaim(OAuth2TokenIntrospectionClaimNames.ISS);
+			assertThat(issClaim).isEqualTo(issuer);
+
+			Object activeClaim = token.getClaim(OAuth2TokenIntrospectionClaimNames.ACTIVE);
+			assertThat(activeClaim).isEqualTo(true);
+		}).doesNotThrowAnyException();
+	}
+
+	@Test
+	void buildWhenIssuerIsValidUriThenAcceptsIssuer() {
+		String issuer = "https://issuer.example.com";
+
+		OAuth2TokenIntrospection token =
+				OAuth2TokenIntrospection.builder(true)
+						.issuer(issuer)
+						.subject("user-123")
+						.build();
+
+		Object issClaim = token.getClaim(OAuth2TokenIntrospectionClaimNames.ISS);
+		assertThat(issClaim).isEqualTo(issuer);
+
+		Object activeClaim = token.getClaim(OAuth2TokenIntrospectionClaimNames.ACTIVE);
+		assertThat(activeClaim).isEqualTo(true);
+	}
+
+	@Test
+	void buildWithMultipleScopes() {
+		OAuth2TokenIntrospection token =
+				OAuth2TokenIntrospection.builder(true)
+						.scope("read")
+						.scope("write")
+						.build();
+
+		List<String> scopes = (List<String>) token.getClaim(OAuth2TokenIntrospectionClaimNames.SCOPE);
+		assertThat(scopes).containsExactly("read", "write");
+	}
+
+	@Test
+	void buildWhenIssuerIsBlankThenThrowsException() {
+		String issuer = "   "; // blank string
+
+		assertThatThrownBy(() ->
+				OAuth2TokenIntrospection.builder(true)
+						.issuer(issuer)
+						.subject("user-123")
+						.build()
+		).isInstanceOf(IllegalArgumentException.class);
+	}
+}