diff --git a/compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/synchronize.kt b/compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/synchronize.kt
new file mode 100644
index 00000000000..7af925daf1b
--- /dev/null
+++ b/compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/synchronize.kt
@@ -0,0 +1,11 @@
+// !LANGUAGE: +ReadDeserializedContracts +UseCallsInPlaceEffect
+
+fun test(lock: Any) {
+    val x: Int
+
+    synchronized(lock) {
+        x = 42
+    }
+
+    x.inc()
+}
\ No newline at end of file
diff --git a/compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/synchronize.txt b/compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/synchronize.txt
new file mode 100644
index 00000000000..68fd635a27b
--- /dev/null
+++ b/compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/synchronize.txt
@@ -0,0 +1,3 @@
+package
+
+public fun test(/*0*/ lock: kotlin.Any): kotlin.Unit
diff --git a/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestWithStdLibGenerated.java b/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestWithStdLibGenerated.java
index 849e36d473f..91f4381ddd4 100644
--- a/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestWithStdLibGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/checkers/DiagnosticsTestWithStdLibGenerated.java
@@ -1192,6 +1192,11 @@ public class DiagnosticsTestWithStdLibGenerated extends AbstractDiagnosticsTestW
             public void testRequire() throws Exception {
                 runTest("compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/require.kt");
             }
+
+            @TestMetadata("synchronize.kt")
+            public void testSynchronize() throws Exception {
+                runTest("compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/synchronize.kt");
+            }
         }
 
         @TestMetadata("compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts")
diff --git a/compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsTestWithStdLibUsingJavacGenerated.java b/compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsTestWithStdLibUsingJavacGenerated.java
index 1909266fe2c..f0a0fe95ef6 100644
--- a/compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsTestWithStdLibUsingJavacGenerated.java
+++ b/compiler/tests/org/jetbrains/kotlin/checkers/javac/DiagnosticsTestWithStdLibUsingJavacGenerated.java
@@ -1192,6 +1192,11 @@ public class DiagnosticsTestWithStdLibUsingJavacGenerated extends AbstractDiagno
             public void testRequire() throws Exception {
                 runTest("compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/require.kt");
             }
+
+            @TestMetadata("synchronize.kt")
+            public void testSynchronize() throws Exception {
+                runTest("compiler/testData/diagnostics/testsWithStdLib/contracts/fromStdlib/synchronize.kt");
+            }
         }
 
         @TestMetadata("compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts")
diff --git a/libraries/stdlib/js/src/kotlin/concurrent.kt b/libraries/stdlib/js/src/kotlin/concurrent.kt
index f0fa4220edb..3ecd424bd6a 100644
--- a/libraries/stdlib/js/src/kotlin/concurrent.kt
+++ b/libraries/stdlib/js/src/kotlin/concurrent.kt
@@ -5,6 +5,8 @@
 
 package kotlin
 
+import kotlin.internal.contracts.*
+
 
 @Deprecated("Use Synchronized annotation from kotlin.jvm package", ReplaceWith("kotlin.jvm.Synchronized"), level = DeprecationLevel.WARNING)
 public typealias Synchronized = kotlin.jvm.Synchronized
@@ -13,4 +15,9 @@ public typealias Synchronized = kotlin.jvm.Synchronized
 public typealias Volatile = kotlin.jvm.Volatile
 
 @kotlin.internal.InlineOnly
-public actual inline fun <R> synchronized(lock: Any, block: () -> R): R = block()
+public actual inline fun <R> synchronized(lock: Any, block: () -> R): R {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+    return block()
+}
diff --git a/libraries/stdlib/jvm/src/kotlin/util/Synchronized.kt b/libraries/stdlib/jvm/src/kotlin/util/Synchronized.kt
index 8301c9785cf..ede49dc2008 100644
--- a/libraries/stdlib/jvm/src/kotlin/util/Synchronized.kt
+++ b/libraries/stdlib/jvm/src/kotlin/util/Synchronized.kt
@@ -7,6 +7,7 @@
 @file:kotlin.jvm.JvmName("StandardKt")
 package kotlin
 
+import kotlin.internal.contracts.*
 import kotlin.jvm.internal.unsafe.*
 
 /**
@@ -14,6 +15,10 @@ import kotlin.jvm.internal.unsafe.*
  */
 @kotlin.internal.InlineOnly
 public actual inline fun <R> synchronized(lock: Any, block: () -> R): R {
+    contract {
+        callsInPlace(block, InvocationKind.EXACTLY_ONCE)
+    }
+
     @Suppress("NON_PUBLIC_CALL_FROM_PUBLIC_INLINE", "INVISIBLE_MEMBER")
     monitorEnter(lock)
     try {