diff --git a/clang/lib/Format/TokenAnnotator.cpp b/clang/lib/Format/TokenAnnotator.cpp
--- a/clang/lib/Format/TokenAnnotator.cpp
+++ b/clang/lib/Format/TokenAnnotator.cpp
@@ -3494,7 +3494,8 @@
 }
 static bool isAllmanLambdaBrace(const FormatToken &Tok) {
   return (Tok.is(tok::l_brace) && Tok.is(BK_Block) &&
-          !Tok.isOneOf(TT_ObjCBlockLBrace, TT_DictLiteral));
+          !Tok.isOneOf(TT_ObjCBlockLBrace, TT_DictLiteral) &&
+          !Tok.Previous->Previous->is(tok::kw_namespace));
 }
 
 static bool isAllmanBraceIncludedBreakableLambda(
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -2589,6 +2589,33 @@
                    Style));
 }
 
+TEST_F(FormatTest, FormatsCompactNamespacesLambdaRegression) {
+  FormatStyle Style = getLLVMStyle();
+  Style.CompactNamespaces = true;
+
+  // Make sure compact namespaces are not confused with lambdas
+  auto NoShortLambdaStyle{Style};
+  NoShortLambdaStyle.AllowShortLambdasOnASingleLine = FormatStyle::SLS_None;
+  EXPECT_EQ("namespace out { namespace in {\n"
+            "}} // namespace out::in",
+            format("namespace out {\n"
+                   "namespace in {\n"
+                   "} // namespace in\n"
+                   "} // namespace out",
+                   NoShortLambdaStyle));
+
+  auto BraceWrappingBeforeLambdaBodyStyle{Style};
+  BraceWrappingBeforeLambdaBodyStyle.BreakBeforeBraces = FormatStyle::BS_Custom;
+  BraceWrappingBeforeLambdaBodyStyle.BraceWrapping.BeforeLambdaBody = true;
+  EXPECT_EQ("namespace out { namespace in {\n"
+            "}} // namespace out::in",
+            format("namespace out {\n"
+                   "namespace in {\n"
+                   "} // namespace in\n"
+                   "} // namespace out",
+                   BraceWrappingBeforeLambdaBodyStyle));
+}
+
 TEST_F(FormatTest, FormatsExternC) {
   verifyFormat("extern \"C\" {\nint a;");
   verifyFormat("extern \"C\" {}");