diff --git a/clang-tools-extra/clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp b/clang-tools-extra/clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp
--- a/clang-tools-extra/clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp
+++ b/clang-tools-extra/clang-tidy/cppcoreguidelines/ProTypeMemberInitCheck.cpp
@@ -61,6 +61,17 @@
   }
 }
 
+void removeFieldInitialized(const FieldDecl *M,
+                            SmallPtrSetImpl<const FieldDecl *> &FieldDecls) {
+  const RecordDecl *R = M->getParent();
+  if (R && R->isUnion()) {
+    // Erase all members in a union if any member of it is initialized.
+    for (const auto *F : R->fields())
+      FieldDecls.erase(F);
+  } else
+    FieldDecls.erase(M);
+}
+
 void removeFieldsInitializedInBody(
     const Stmt &Stmt, ASTContext &Context,
     SmallPtrSetImpl<const FieldDecl *> &FieldDecls) {
@@ -70,7 +81,7 @@
                 hasLHS(memberExpr(member(fieldDecl().bind("fieldDecl")))))),
             Stmt, Context);
   for (const auto &Match : Matches)
-    FieldDecls.erase(Match.getNodeAs<FieldDecl>("fieldDecl"));
+    removeFieldInitialized(Match.getNodeAs<FieldDecl>("fieldDecl"), FieldDecls);
 }
 
 StringRef getName(const FieldDecl *Field) { return Field->getName(); }
@@ -418,13 +429,20 @@
 
   // Gather all fields (direct and indirect) that need to be initialized.
   SmallPtrSet<const FieldDecl *, 16> FieldsToInit;
-  forEachField(ClassDecl, ClassDecl.fields(), [&](const FieldDecl *F) {
+  bool AnyMemberHasInitPerUnion = false;
+  forEachFieldWithFilter(ClassDecl, ClassDecl.fields(),
+                         AnyMemberHasInitPerUnion, [&](const FieldDecl *F) {
     if (IgnoreArrays && F->getType()->isArrayType())
       return;
+    if (F->hasInClassInitializer() && F->getParent()->isUnion()) {
+      AnyMemberHasInitPerUnion = true;
+      removeFieldInitialized(F, FieldsToInit);
+    }
     if (!F->hasInClassInitializer() &&
         utils::type_traits::isTriviallyDefaultConstructible(F->getType(),
                                                             Context) &&
-        !isEmpty(Context, F->getType()) && !F->isUnnamedBitfield())
+        !isEmpty(Context, F->getType()) && !F->isUnnamedBitfield() &&
+        !AnyMemberHasInitPerUnion)
       FieldsToInit.insert(F);
   });
   if (FieldsToInit.empty())
@@ -437,7 +455,7 @@
       if (Init->isAnyMemberInitializer() && Init->isWritten()) {
         if (IsUnion)
           return; // We can only initialize one member of a union.
-        FieldsToInit.erase(Init->getAnyMember());
+        removeFieldInitialized(Init->getAnyMember(), FieldsToInit);
       }
     }
     removeFieldsInitializedInBody(*Ctor->getBody(), Context, FieldsToInit);
@@ -478,7 +496,7 @@
   // Collect all fields but only suggest a fix for the first member of unions,
   // as initializing more than one union member is an error.
   SmallPtrSet<const FieldDecl *, 16> FieldsToFix;
-  bool AnyMemberHasInitPerUnion = false;
+  AnyMemberHasInitPerUnion = false;
   forEachFieldWithFilter(ClassDecl, ClassDecl.fields(),
                          AnyMemberHasInitPerUnion, [&](const FieldDecl *F) {
     if (!FieldsToInit.count(F))
diff --git a/clang-tools-extra/docs/ReleaseNotes.rst b/clang-tools-extra/docs/ReleaseNotes.rst
--- a/clang-tools-extra/docs/ReleaseNotes.rst
+++ b/clang-tools-extra/docs/ReleaseNotes.rst
@@ -120,6 +120,11 @@
   <clang-tidy/checks/bugprone/signal-handler>` check. Partial
   support for C++14 signal handler rules was added. Bug report generation was
   improved.
+
+- Fixed a false positive in :doc:`cppcoreguidelines-pro-type-member-init
+  <clang-tidy/checks/cppcoreguidelines/pro-type-member-init>` when warnings
+  would be emitted for uninitialized members of an anonymous union despite
+  there being an initializer for one of the other members.
   
 - Improved `modernize-use-emplace <clang-tidy/checks/modernize/use-emplace.html>`_ check.
 
diff --git a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/pro-type-member-init.cpp b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/pro-type-member-init.cpp
--- a/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/pro-type-member-init.cpp
+++ b/clang-tools-extra/test/clang-tidy/checkers/cppcoreguidelines/pro-type-member-init.cpp
@@ -552,3 +552,30 @@
   int A;
   // CHECK-FIXES-NOT: int A{};
 };
+
+struct S1 {
+  S1() {}
+  union {
+    int a = 0;
+    int b;
+  };
+};
+
+struct S2 {
+  S2() : a{} {}
+  union {
+    int a;
+    int b;
+  };
+};
+
+struct S3 {
+  S3() {}
+  // CHECK-MESSAGES: :[[@LINE-1]]:3: warning: constructor does not initialize these fields: A [cppcoreguidelines-pro-type-member-init]
+  int A;
+  // CHECK-FIXES: int A{};
+  union {
+    int B;
+    int C = 0;
+  };
+};