diff --git a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
index 823945b597..695c1277dc 100644
--- a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
+++ b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
@@ -344,6 +344,9 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public void setLocale(Locale locale) {
+		if (locale == null) {
+			return;
+		}
 		this.locale = locale;
 		doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, locale.toLanguageTag(), true);
 	}
diff --git a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
index 907a5cfac9..08fda3f723 100644
--- a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
+++ b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
@@ -85,6 +85,13 @@ class MockHttpServletResponseTests {
 		assertThat(response.containsHeader(headerName)).isFalse();
 	}
 
+	@Test  // gh-26493
+	void setLocaleWithNullValue() {
+		assertThat(response.getLocale()).isEqualTo(Locale.getDefault());
+		response.setLocale(null);
+		assertThat(response.getLocale()).isEqualTo(Locale.getDefault());
+	}
+
 	@Test
 	void setContentType() {
 		String contentType = "test/plain";
diff --git a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
index 5af669a25e..0459dcb7db 100644
--- a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
+++ b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
@@ -344,6 +344,9 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public void setLocale(Locale locale) {
+		if (locale == null) {
+			return;
+		}
 		this.locale = locale;
 		doAddHeaderValue(HttpHeaders.CONTENT_LANGUAGE, locale.toLanguageTag(), true);
 	}