diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9e060eef42..a498f621f8 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -30,6 +30,7 @@ via the option `swizzleClassNameExclude`.
 - Speed up getBinaryImages (#4435) for finishing transactions and capturing events
 - Align SDK dispatch queue names (#4442) to start with `io.sentry`
 - Use UInts in envelope deserialization (#4441)
+- Make `SentrySDK.replay.start()` thread safe (#4455)
 
 ## 8.38.0
 
diff --git a/Sources/Sentry/SentryReplayApi.m b/Sources/Sentry/SentryReplayApi.m
index 3aad160229..d28c323ec3 100644
--- a/Sources/Sentry/SentryReplayApi.m
+++ b/Sources/Sentry/SentryReplayApi.m
@@ -3,6 +3,7 @@
 #if SENTRY_TARGET_REPLAY_SUPPORTED
 
 #    import "SentryHub+Private.h"
+#    import "SentryInternalCDefines.h"
 #    import "SentryOptions+Private.h"
 #    import "SentrySDK+Private.h"
 #    import "SentrySessionReplayIntegration+Private.h"
@@ -37,21 +38,30 @@ - (void)resume
     [replayIntegration resume];
 }
 
-- (void)start
+- (void)start SENTRY_DISABLE_THREAD_SANITIZER("double-checked lock produce false alarms")
 {
     SentrySessionReplayIntegration *replayIntegration
         = (SentrySessionReplayIntegration *)[SentrySDK.currentHub
             getInstalledIntegration:SentrySessionReplayIntegration.class];
 
+    // Start could be misused and called multiple times, causing it to
+    // be initialized more than once before being installed.
+    // Synchronizing it will prevent this problem.
     if (replayIntegration == nil) {
-        SentryOptions *currentOptions = SentrySDK.currentHub.client.options;
-        replayIntegration =
-            [[SentrySessionReplayIntegration alloc] initForManualUse:currentOptions];
+        @synchronized(self) {
+            replayIntegration = (SentrySessionReplayIntegration *)[SentrySDK.currentHub
+                getInstalledIntegration:SentrySessionReplayIntegration.class];
+            if (replayIntegration == nil) {
+                SentryOptions *currentOptions = SentrySDK.currentHub.client.options;
+                replayIntegration =
+                    [[SentrySessionReplayIntegration alloc] initForManualUse:currentOptions];
 
-        [SentrySDK.currentHub addInstalledIntegration:replayIntegration
-                                                 name:NSStringFromClass(SentrySessionReplay.class)];
+                [SentrySDK.currentHub
+                    addInstalledIntegration:replayIntegration
+                                       name:NSStringFromClass(SentrySessionReplay.class)];
+            }
+        }
     }
-
     [replayIntegration start];
 }