diff --git a/spring-web/src/main/java/org/springframework/http/server/ServletRequestHeadersAdapter.java b/spring-web/src/main/java/org/springframework/http/server/ServletRequestHeadersAdapter.java
index cee16887c10..04a57b08812 100644
--- a/spring-web/src/main/java/org/springframework/http/server/ServletRequestHeadersAdapter.java
+++ b/spring-web/src/main/java/org/springframework/http/server/ServletRequestHeadersAdapter.java
@@ -305,12 +305,13 @@ final class ServletRequestHeadersAdapter implements MultiValueMap<String, String
 			if (this.overrideMap == null) {
 				return this.delegate.size();
 			}
-			Set<String> set = new LinkedHashSet<>();
+			int size = this.overrideMap.size();
 			for (String name : this.delegate.keySet()) {
-				set.add(name.toLowerCase(Locale.ROOT));
+				if (!this.overrideMap.containsKey(name)) {
+					size++;
+				}
 			}
-			this.overrideMap.keySet().forEach(key -> set.add(key.toLowerCase(Locale.ROOT)));
-			return set.size();
+			return size;
 		}
 
 		@Override
@@ -375,7 +376,12 @@ final class ServletRequestHeadersAdapter implements MultiValueMap<String, String
 		@Override
 		public Set<String> keySet() {
 			if (this.overrideMap != null) {
-				Set<String> set = new LinkedHashSet<>(this.delegate.keySet());
+				Set<String> set = new LinkedHashSet<>();
+				for (String name : this.delegate.keySet()) {
+					if (!this.overrideMap.containsKey(name)) {
+						set.add(name);
+					}
+				}
 				set.addAll(this.overrideMap.keySet());
 				return set;
 			}
diff --git a/spring-web/src/test/java/org/springframework/http/server/ServletRequestHeadersAdapterTests.java b/spring-web/src/test/java/org/springframework/http/server/ServletRequestHeadersAdapterTests.java
new file mode 100644
index 00000000000..560d5410efe
--- /dev/null
+++ b/spring-web/src/test/java/org/springframework/http/server/ServletRequestHeadersAdapterTests.java
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2002-present the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.http.server;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.util.MultiValueMap;
+import org.springframework.web.testfixture.servlet.MockHttpServletRequest;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Unit tests for {@link ServletRequestHeadersAdapter}.
+ */
+public class ServletRequestHeadersAdapterTests {
+
+	private final MockHttpServletRequest request = new MockHttpServletRequest();
+
+	private final MultiValueMap<String, String> headersAdapter = ServletRequestHeadersAdapter.create(request);
+
+
+	@Test // gh-36418
+	void caseSensitiveOverride() {
+		request.addHeader("foo", "value");
+		headersAdapter.set("Foo", "override value");
+
+		assertThat(headersAdapter.size()).isEqualTo(1);
+		assertThat(headersAdapter.keySet()).containsExactly("Foo");
+		assertThat(headersAdapter.getFirst("foo")).isEqualTo("override value");
+		assertThat(headersAdapter.get("foo")).containsExactly("override value");
+	}
+
+}