diff --git a/clang-tools-extra/clangd/IncludeCleaner.cpp b/clang-tools-extra/clangd/IncludeCleaner.cpp
--- a/clang-tools-extra/clangd/IncludeCleaner.cpp
+++ b/clang-tools-extra/clangd/IncludeCleaner.cpp
@@ -39,6 +39,13 @@
     return true;
   }
 
+  bool VisitFunctionDecl(FunctionDecl *FD) {
+    // Function definition will require redeclarations to be included.
+    if (FD == FD->getDefinition())
+      add(FD);
+    return true;
+  }
+
   bool VisitCXXConstructExpr(CXXConstructExpr *CCE) {
     add(CCE->getConstructor());
     return true;
diff --git a/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp b/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
--- a/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
+++ b/clang-tools-extra/clangd/unittests/IncludeCleanerTests.cpp
@@ -79,6 +79,19 @@
           "struct ^X { ^X(int) {} int ^foo(); };",
           "auto x = X(42); auto y = x.foo();",
       },
+      // Function
+      {
+          "void ^foo();",
+          "void foo() {}",
+      },
+      {
+          "void foo() {}",
+          "void foo();",
+      },
+      {
+          "inline void ^foo() {}",
+          "void bar() { foo(); }",
+      },
       // Static function
       {
           "struct ^X { static bool ^foo(); }; bool X::^foo() {}",