diff --git a/spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java b/spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java
index e5f31194827..b45c9ddeb82 100644
--- a/spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java
+++ b/spring-web/src/main/java/org/springframework/http/converter/StringHttpMessageConverter.java
@@ -43,6 +43,8 @@ import org.springframework.util.StreamUtils;
  */
 public class StringHttpMessageConverter extends AbstractHttpMessageConverter<String> {
 
+	private static final MediaType APPLICATION_PLUS_JSON = new MediaType("application", "*+json");
+
 	/**
 	 * The default charset used by the converter.
 	 */
@@ -104,7 +106,9 @@ public class StringHttpMessageConverter extends AbstractHttpMessageConverter<Str
 	@Override
 	protected void addDefaultHeaders(HttpHeaders headers, String s, @Nullable MediaType type) throws IOException {
 		if (headers.getContentType() == null ) {
-			if (type != null && type.isConcrete() && type.isCompatibleWith(MediaType.APPLICATION_JSON)) {
+			if (type != null && type.isConcrete() &&
+					(type.isCompatibleWith(MediaType.APPLICATION_JSON) ||
+					type.isCompatibleWith(APPLICATION_PLUS_JSON))) {
 				// Prevent charset parameter for JSON..
 				headers.setContentType(type);
 			}
@@ -144,7 +148,8 @@ public class StringHttpMessageConverter extends AbstractHttpMessageConverter<Str
 			if (charset != null) {
 				return charset;
 			}
-			else if (contentType.isCompatibleWith(MediaType.APPLICATION_JSON)) {
+			else if (contentType.isCompatibleWith(MediaType.APPLICATION_JSON) ||
+					contentType.isCompatibleWith(APPLICATION_PLUS_JSON)) {
 				// Matching to AbstractJackson2HttpMessageConverter#DEFAULT_CHARSET
 				return StandardCharsets.UTF_8;
 			}
diff --git a/spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java b/spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java
index 5ee6df9b009..3323dbdb350 100644
--- a/spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java
+++ b/spring-web/src/test/java/org/springframework/http/converter/StringHttpMessageConverterTests.java
@@ -80,6 +80,16 @@ public class StringHttpMessageConverterTests {
 		assertThat(result).as("Invalid result").isEqualTo(body);
 	}
 
+	@Test // gh-25328
+	public void readJsonApi() throws IOException {
+		String body = "{\"result\":\"\u0414\u0410\"}";
+		MockHttpInputMessage inputMessage = new MockHttpInputMessage(body.getBytes(StandardCharsets.UTF_8));
+		inputMessage.getHeaders().setContentType(new MediaType("application", "vnd.api.v1+json"));
+		String result = this.converter.read(String.class, inputMessage);
+
+		assertThat(result).as("Invalid result").isEqualTo(body);
+	}
+
 	@Test
 	public void writeDefaultCharset() throws IOException {
 		String body = "H\u00e9llo W\u00f6rld";
@@ -94,7 +104,7 @@ public class StringHttpMessageConverterTests {
 
 	@Test // gh-24123
 	public void writeJson() throws IOException {
-		String body = "{\"foo\":\"bar\"}";
+		String body = "{\"føø\":\"bår\"}";
 		this.converter.write(body, MediaType.APPLICATION_JSON, this.outputMessage);
 
 		HttpHeaders headers = this.outputMessage.getHeaders();
@@ -104,6 +114,19 @@ public class StringHttpMessageConverterTests {
 		assertThat(headers.getAcceptCharset().isEmpty()).isTrue();
 	}
 
+	@Test // gh-25328
+	public void writeJsonApi() throws IOException {
+		String body = "{\"føø\":\"bår\"}";
+		MediaType contentType = new MediaType("application", "vnd.api.v1+json");
+		this.converter.write(body, contentType, this.outputMessage);
+
+		HttpHeaders headers = this.outputMessage.getHeaders();
+		assertThat(this.outputMessage.getBodyAsString(StandardCharsets.UTF_8)).isEqualTo(body);
+		assertThat(headers.getContentType()).isEqualTo(contentType);
+		assertThat(headers.getContentLength()).isEqualTo(body.getBytes(StandardCharsets.UTF_8).length);
+		assertThat(headers.getAcceptCharset().isEmpty()).isTrue();
+	}
+
 	@Test
 	public void writeUTF8() throws IOException {
 		String body = "H\u00e9llo W\u00f6rld";