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 d1365943a5..7abccf5a06 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
@@ -100,6 +100,18 @@ public class StringHttpMessageConverter extends AbstractHttpMessageConverter<Str
 		return (long) str.getBytes(charset).length;
 	}
 
+
+	@Override
+	protected void addDefaultHeaders(HttpHeaders headers, String s, @Nullable MediaType mediaType) throws IOException {
+		if (headers.getContentType() == null ) {
+			if (mediaType != null && mediaType.isCompatibleWith(MediaType.APPLICATION_JSON)) {
+				// Prevent charset parameter for JSON..
+				headers.setContentType(mediaType);
+			}
+		}
+		super.addDefaultHeaders(headers, s, mediaType);
+	}
+
 	@Override
 	protected void writeInternal(String str, HttpOutputMessage outputMessage) throws IOException {
 		HttpHeaders headers = outputMessage.getHeaders();
@@ -130,6 +142,10 @@ public class StringHttpMessageConverter extends AbstractHttpMessageConverter<Str
 		if (contentType != null && contentType.getCharset() != null) {
 			return contentType.getCharset();
 		}
+		else if (contentType != null && contentType.isCompatibleWith(MediaType.APPLICATION_JSON)) {
+			// Matching to AbstractJackson2HttpMessageConverter#DEFAULT_CHARSET
+			return StandardCharsets.UTF_8;
+		}
 		else {
 			Charset charset = getDefaultCharset();
 			Assert.state(charset != null, "No default charset");
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 8c66bc4de9..c0e8792d33 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
@@ -70,6 +70,16 @@ public class StringHttpMessageConverterTests {
 		assertThat(result).as("Invalid result").isEqualTo(body);
 	}
 
+	@Test // gh-24123
+	public void readJson() throws IOException {
+		String body = "{\"result\":\"\u0414\u0410\"}";
+		MockHttpInputMessage inputMessage = new MockHttpInputMessage(body.getBytes(StandardCharsets.UTF_8));
+		inputMessage.getHeaders().setContentType(MediaType.APPLICATION_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";
@@ -82,6 +92,18 @@ public class StringHttpMessageConverterTests {
 		assertThat(headers.getAcceptCharset().isEmpty()).isTrue();
 	}
 
+	@Test // gh-24123
+	public void writeJson() throws IOException {
+		String body = "{\"foo\":\"bar\"}";
+		this.converter.write(body, MediaType.APPLICATION_JSON, this.outputMessage);
+
+		HttpHeaders headers = this.outputMessage.getHeaders();
+		assertThat(this.outputMessage.getBodyAsString(StandardCharsets.UTF_8)).isEqualTo(body);
+		assertThat(headers.getContentType()).isEqualTo(MediaType.APPLICATION_JSON);
+		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";
diff --git a/spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ServletAnnotationControllerHandlerMethodTests.java b/spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ServletAnnotationControllerHandlerMethodTests.java
index 502b11ab5c..565f785d29 100644
--- a/spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ServletAnnotationControllerHandlerMethodTests.java
+++ b/spring-webmvc/src/test/java/org/springframework/web/servlet/mvc/method/annotation/ServletAnnotationControllerHandlerMethodTests.java
@@ -1011,7 +1011,7 @@ public class ServletAnnotationControllerHandlerMethodTests extends AbstractServl
 		request.addHeader("Accept", "application/json, text/javascript, */*");
 		MockHttpServletResponse response = new MockHttpServletResponse();
 		getServlet().service(request, response);
-		assertThat(response.getHeader("Content-Type")).as("Invalid content-type").isEqualTo("application/json;charset=ISO-8859-1");
+		assertThat(response.getHeader("Content-Type")).as("Invalid content-type").isEqualTo("application/json");
 	}
 
 	@Test
@@ -1548,7 +1548,7 @@ public class ServletAnnotationControllerHandlerMethodTests extends AbstractServl
 		getServlet().service(request, response);
 
 		assertThat(response.getStatus()).isEqualTo(200);
-		assertThat(response.getHeader("Content-Type")).isEqualTo("application/json;charset=ISO-8859-1");
+		assertThat(response.getHeader("Content-Type")).isEqualTo("application/json");
 		assertThat(response.getContentAsString()).isEqualTo("homeJson");
 	}