diff --git a/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java b/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java
index 214139d50165..85838a79288a 100644
--- a/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java
+++ b/spring-core/src/main/java/org/springframework/util/MimeTypeUtils.java
@@ -23,8 +23,8 @@
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Iterator;
-import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Random;
 import java.util.function.BiPredicate;
@@ -246,7 +246,9 @@ else if (ch == '"' && (nextIndex == 0 || mimeType.charAt(nextIndex - 1) != '\\')
 			String parameter = mimeType.substring(index + 1, nextIndex).trim();
 			if (parameter.length() > 0) {
 				if (parameters == null) {
-					parameters = new LinkedHashMap<>(4);
+					// Parameter names are case-insensitive, so use a case-insensitive
+					// map in order to reject duplicates that differ only in case.
+					parameters = new LinkedCaseInsensitiveMap<>(4, Locale.ROOT);
 				}
 				int eqIndex = parameter.indexOf('=');
 				if (eqIndex >= 0) {
diff --git a/spring-core/src/test/java/org/springframework/util/MimeTypeTests.java b/spring-core/src/test/java/org/springframework/util/MimeTypeTests.java
index 29cf7119b508..13ba1109c6f7 100644
--- a/spring-core/src/test/java/org/springframework/util/MimeTypeTests.java
+++ b/spring-core/src/test/java/org/springframework/util/MimeTypeTests.java
@@ -124,6 +124,13 @@ void rejectsDuplicateParameter() {
 				.hasMessageContaining("Invalid mime type \"text/plain;dupe=\"1\";dupe=\"2\"\": duplicate parameter 'dupe=\"2\"'");
 	}
 
+	@Test
+	void rejectsDuplicateParameterWithDifferentCase() {
+		String s = "text/plain;dupe=\"1\";DUPE=\"2\"";
+		assertThatThrownBy(() -> MimeType.valueOf(s)).isInstanceOf(InvalidMimeTypeException.class)
+				.hasMessageContaining("Invalid mime type \"text/plain;dupe=\"1\";DUPE=\"2\"\": duplicate parameter 'DUPE=\"2\"'");
+	}
+
 	@Test
 	void withConversionService() {
 		ConversionService conversionService = new DefaultConversionService();