diff --git a/spring-web/src/main/java/org/springframework/web/server/WebSession.java b/spring-web/src/main/java/org/springframework/web/server/WebSession.java
index a1c823393f9..3142dbf4873 100644
--- a/spring-web/src/main/java/org/springframework/web/server/WebSession.java
+++ b/spring-web/src/main/java/org/springframework/web/server/WebSession.java
@@ -100,20 +100,6 @@ public interface WebSession {
 	 */
 	boolean isStarted();
 
-	/**
-	 * Generate a new id for the session and update the underlying session
-	 * storage to reflect the new id. After a successful call {@link #getId()}
-	 * reflects the new session id.
-	 * @return completion notification (success or error)
-	 */
-	Mono<Void> changeSessionId();
-
-	/**
-	 * Invalidate the current session and clear session storage.
-	 * @return completion notification (success or error)
-	 */
-	Mono<Void> invalidate();
-
 	/**
 	 * Save the session through the {@code WebSessionStore} as follows:
 	 * <ul>
@@ -131,6 +117,20 @@ public interface WebSession {
 	 */
 	Mono<Void> save();
 
+	/**
+	 * Generate a new id for the session and update the underlying session
+	 * storage to reflect the new id. After a successful call {@link #getId()}
+	 * reflects the new session id.
+	 * @return completion notification (success or error)
+	 */
+	Mono<Void> changeSessionId();
+
+	/**
+	 * Invalidate the current session and clear session storage.
+	 * @return completion notification (success or error)
+	 */
+	Mono<Void> invalidate();
+
 	/**
 	 * Return {@code true} if the session expired after {@link #getMaxIdleTime()
 	 * maxIdleTime} elapsed.
diff --git a/spring-web/src/main/java/org/springframework/web/server/session/InMemoryWebSessionStore.java b/spring-web/src/main/java/org/springframework/web/server/session/InMemoryWebSessionStore.java
index 6ec24eaacb3..9b226edadc0 100644
--- a/spring-web/src/main/java/org/springframework/web/server/session/InMemoryWebSessionStore.java
+++ b/spring-web/src/main/java/org/springframework/web/server/session/InMemoryWebSessionStore.java
@@ -207,6 +207,7 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 
 		private final AtomicReference<State> state = new AtomicReference<>(State.NEW);
 
+		private final Lock lock = new ReentrantLock();
 
 		public InMemoryWebSession(Instant creationTime, Duration maxIdleTime) {
 			this.creationTime = creationTime;
@@ -256,29 +257,6 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 			return this.state.get().equals(State.STARTED) || !getAttributes().isEmpty();
 		}
 
-		@Override
-		public Mono<Void> changeSessionId() {
-			return Mono.<Void>defer(() -> {
-						String currentId = this.id.get();
-						InMemoryWebSessionStore.this.sessions.remove(currentId);
-						String newId = String.valueOf(idGenerator.generateId());
-						this.id.set(newId);
-						InMemoryWebSessionStore.this.sessions.put(this.id.get(), this);
-						return Mono.empty();
-					})
-					.subscribeOn(Schedulers.boundedElastic())
-					.publishOn(Schedulers.parallel())
-					.then();
-		}
-
-		@Override
-		public Mono<Void> invalidate() {
-			this.state.set(State.EXPIRED);
-			getAttributes().clear();
-			InMemoryWebSessionStore.this.sessions.remove(this.id.get());
-			return Mono.empty();
-		}
-
 		@Override
 		@SuppressWarnings("NullAway") // Dataflow analysis limitation
 		public Mono<Void> save() {
@@ -292,11 +270,19 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 
 			if (isStarted()) {
 				// Save
-				InMemoryWebSessionStore.this.sessions.put(this.id.get(), this);
+				if (InMemoryWebSessionStore.this.sessions.get(getId()) == null) {
+					this.lock.lock();
+					try {
+						InMemoryWebSessionStore.this.sessions.putIfAbsent(getId(), this);
+					}
+					finally {
+						this.lock.unlock();
+					}
+				}
 
 				// Unless it was invalidated
 				if (this.state.get().equals(State.EXPIRED)) {
-					InMemoryWebSessionStore.this.sessions.remove(this.id.get());
+					InMemoryWebSessionStore.this.sessions.remove(getId());
 					return Mono.error(new IllegalStateException("Session was invalidated"));
 				}
 			}
@@ -307,12 +293,41 @@ public class InMemoryWebSessionStore implements WebSessionStore {
 		private void checkMaxSessionsLimit() {
 			if (sessions.size() >= maxSessions) {
 				expiredSessionChecker.removeExpiredSessions(clock.instant());
-				if (sessions.size() >= maxSessions && !sessions.containsKey(this.id.get())) {
+				if (sessions.size() >= maxSessions && !sessions.containsKey(getId())) {
 					throw new IllegalStateException("Max sessions limit reached: " + sessions.size());
 				}
 			}
 		}
 
+		@Override
+		public Mono<Void> changeSessionId() {
+			return Mono.<Void>defer(() -> {
+						this.lock.lock();
+						try {
+							String oldId = getId();
+							String newId = String.valueOf(idGenerator.generateId());
+							InMemoryWebSessionStore.this.sessions.remove(oldId);
+							InMemoryWebSessionStore.this.sessions.put(newId, this);
+							this.id.set(newId);
+						}
+						finally {
+							this.lock.unlock();
+						}
+						return Mono.empty();
+					})
+					.subscribeOn(Schedulers.boundedElastic())
+					.publishOn(Schedulers.parallel())
+					.then();
+		}
+
+		@Override
+		public Mono<Void> invalidate() {
+			this.state.set(State.EXPIRED);
+			getAttributes().clear();
+			InMemoryWebSessionStore.this.sessions.remove(getId());
+			return Mono.empty();
+		}
+
 		@Override
 		public boolean isExpired() {
 			return isExpired(clock.instant());