diff --git a/clang/lib/ExtractAPI/Serialization/SymbolGraphSerializer.cpp b/clang/lib/ExtractAPI/Serialization/SymbolGraphSerializer.cpp
--- a/clang/lib/ExtractAPI/Serialization/SymbolGraphSerializer.cpp
+++ b/clang/lib/ExtractAPI/Serialization/SymbolGraphSerializer.cpp
@@ -467,6 +467,11 @@
   if (Record.Availability.isUnconditionallyUnavailable())
     return true;
 
+  // Filter out symbols prefixed with an underscored as they are understood to
+  // be symbols clients should not use.
+  if (Record.Name.startswith("_"))
+    return true;
+
   return false;
 }
 
diff --git a/clang/test/ExtractAPI/underscored.c b/clang/test/ExtractAPI/underscored.c
new file mode 100644
--- /dev/null
+++ b/clang/test/ExtractAPI/underscored.c
@@ -0,0 +1,396 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: sed -e "s@INPUT_DIR@%{/t:regex_replacement}@g" \
+// RUN: %t/reference.output.json.in >> %t/reference.output.json
+// RUN: %clang_cc1 -extract-api -triple arm64-apple-macosx \
+// RUN:   -x c-header %t/input.h -o %t/output.json -verify
+
+// Generator version is not consistent across test runs, normalize it.
+// RUN: sed -e "s@\"generator\": \".*\"@\"generator\": \"?\"@g" \
+// RUN: %t/output.json >> %t/output-normalized.json
+// RUN: diff %t/reference.output.json %t/output-normalized.json
+
+//--- input.h
+// expected-no-diagnostics
+
+// Global record
+int _HiddenGlobal;
+int exposed_global;
+
+// Record type
+struct _HiddenRecord {
+  int a;
+};
+
+struct ExposedRecord {
+  int a;
+};
+
+// Typedef
+typedef struct {} _HiddenTypedef;
+typedef int ExposedTypedef;
+typedef _HiddenTypedef ExposedTypedefToHidden;
+
+// Macros
+#define _HIDDEN_MACRO 5
+#define EXPOSED_MACRO 5
+
+// Symbols that start with '_' should not appear in the reference output
+//--- reference.output.json.in
+{
+  "metadata": {
+    "formatVersion": {
+      "major": 0,
+      "minor": 5,
+      "patch": 3
+    },
+    "generator": "?"
+  },
+  "module": {
+    "name": "",
+    "platform": {
+      "architecture": "arm64",
+      "operatingSystem": {
+        "minimumVersion": {
+          "major": 11,
+          "minor": 0,
+          "patch": 0
+        },
+        "name": "macosx"
+      },
+      "vendor": "apple"
+    }
+  },
+  "relationships": [
+    {
+      "kind": "memberOf",
+      "source": "c:@S@ExposedRecord@FI@a",
+      "target": "c:@S@ExposedRecord"
+    }
+  ],
+  "symbols": [
+    {
+      "accessLevel": "public",
+      "declarationFragments": [
+        {
+          "kind": "typeIdentifier",
+          "preciseIdentifier": "c:I",
+          "spelling": "int"
+        },
+        {
+          "kind": "text",
+          "spelling": " "
+        },
+        {
+          "kind": "identifier",
+          "spelling": "exposed_global"
+        }
+      ],
+      "identifier": {
+        "interfaceLanguage": "c",
+        "precise": "c:@exposed_global"
+      },
+      "kind": {
+        "displayName": "Global Variable",
+        "identifier": "c.var"
+      },
+      "location": {
+        "position": {
+          "character": 5,
+          "line": 5
+        },
+        "uri": "file://INPUT_DIR/input.h"
+      },
+      "names": {
+        "navigator": [
+          {
+            "kind": "identifier",
+            "spelling": "exposed_global"
+          }
+        ],
+        "subHeading": [
+          {
+            "kind": "identifier",
+            "spelling": "exposed_global"
+          }
+        ],
+        "title": "exposed_global"
+      },
+      "pathComponents": [
+        "exposed_global"
+      ]
+    },
+    {
+      "accessLevel": "public",
+      "declarationFragments": [
+        {
+          "kind": "keyword",
+          "spelling": "struct"
+        },
+        {
+          "kind": "text",
+          "spelling": " "
+        },
+        {
+          "kind": "identifier",
+          "spelling": "ExposedRecord"
+        }
+      ],
+      "identifier": {
+        "interfaceLanguage": "c",
+        "precise": "c:@S@ExposedRecord"
+      },
+      "kind": {
+        "displayName": "Structure",
+        "identifier": "c.struct"
+      },
+      "location": {
+        "position": {
+          "character": 8,
+          "line": 12
+        },
+        "uri": "file://INPUT_DIR/input.h"
+      },
+      "names": {
+        "navigator": [
+          {
+            "kind": "identifier",
+            "spelling": "ExposedRecord"
+          }
+        ],
+        "subHeading": [
+          {
+            "kind": "identifier",
+            "spelling": "ExposedRecord"
+          }
+        ],
+        "title": "ExposedRecord"
+      },
+      "pathComponents": [
+        "ExposedRecord"
+      ]
+    },
+    {
+      "accessLevel": "public",
+      "declarationFragments": [
+        {
+          "kind": "typeIdentifier",
+          "preciseIdentifier": "c:I",
+          "spelling": "int"
+        },
+        {
+          "kind": "text",
+          "spelling": " "
+        },
+        {
+          "kind": "identifier",
+          "spelling": "a"
+        }
+      ],
+      "identifier": {
+        "interfaceLanguage": "c",
+        "precise": "c:@S@ExposedRecord@FI@a"
+      },
+      "kind": {
+        "displayName": "Instance Property",
+        "identifier": "c.property"
+      },
+      "location": {
+        "position": {
+          "character": 7,
+          "line": 13
+        },
+        "uri": "file://INPUT_DIR/input.h"
+      },
+      "names": {
+        "navigator": [
+          {
+            "kind": "identifier",
+            "spelling": "a"
+          }
+        ],
+        "subHeading": [
+          {
+            "kind": "identifier",
+            "spelling": "a"
+          }
+        ],
+        "title": "a"
+      },
+      "pathComponents": [
+        "ExposedRecord",
+        "a"
+      ]
+    },
+    {
+      "accessLevel": "public",
+      "declarationFragments": [
+        {
+          "kind": "keyword",
+          "spelling": "#define"
+        },
+        {
+          "kind": "text",
+          "spelling": " "
+        },
+        {
+          "kind": "identifier",
+          "spelling": "EXPOSED_MACRO"
+        }
+      ],
+      "identifier": {
+        "interfaceLanguage": "c",
+        "precise": "c:input.h@335@macro@EXPOSED_MACRO"
+      },
+      "kind": {
+        "displayName": "Macro",
+        "identifier": "c.macro"
+      },
+      "location": {
+        "position": {
+          "character": 9,
+          "line": 23
+        },
+        "uri": "file://INPUT_DIR/input.h"
+      },
+      "names": {
+        "navigator": [
+          {
+            "kind": "identifier",
+            "spelling": "EXPOSED_MACRO"
+          }
+        ],
+        "subHeading": [
+          {
+            "kind": "identifier",
+            "spelling": "EXPOSED_MACRO"
+          }
+        ],
+        "title": "EXPOSED_MACRO"
+      },
+      "pathComponents": [
+        "EXPOSED_MACRO"
+      ]
+    },
+    {
+      "accessLevel": "public",
+      "declarationFragments": [
+        {
+          "kind": "keyword",
+          "spelling": "typedef"
+        },
+        {
+          "kind": "text",
+          "spelling": " "
+        },
+        {
+          "kind": "typeIdentifier",
+          "preciseIdentifier": "c:I",
+          "spelling": "int"
+        },
+        {
+          "kind": "text",
+          "spelling": " "
+        },
+        {
+          "kind": "identifier",
+          "spelling": "ExposedTypedef"
+        }
+      ],
+      "identifier": {
+        "interfaceLanguage": "c",
+        "precise": "c:input.h@T@ExposedTypedef"
+      },
+      "kind": {
+        "displayName": "Type Alias",
+        "identifier": "c.typealias"
+      },
+      "location": {
+        "position": {
+          "character": 13,
+          "line": 18
+        },
+        "uri": "file://INPUT_DIR/input.h"
+      },
+      "names": {
+        "navigator": [
+          {
+            "kind": "identifier",
+            "spelling": "ExposedTypedef"
+          }
+        ],
+        "subHeading": [
+          {
+            "kind": "identifier",
+            "spelling": "ExposedTypedef"
+          }
+        ],
+        "title": "ExposedTypedef"
+      },
+      "pathComponents": [
+        "ExposedTypedef"
+      ],
+      "type": "c:I"
+    },
+    {
+      "accessLevel": "public",
+      "declarationFragments": [
+        {
+          "kind": "keyword",
+          "spelling": "typedef"
+        },
+        {
+          "kind": "text",
+          "spelling": " "
+        },
+        {
+          "kind": "typeIdentifier",
+          "preciseIdentifier": "c:@SA@_HiddenTypedef",
+          "spelling": "_HiddenTypedef"
+        },
+        {
+          "kind": "text",
+          "spelling": " "
+        },
+        {
+          "kind": "identifier",
+          "spelling": "ExposedTypedefToHidden"
+        }
+      ],
+      "identifier": {
+        "interfaceLanguage": "c",
+        "precise": "c:input.h@T@ExposedTypedefToHidden"
+      },
+      "kind": {
+        "displayName": "Type Alias",
+        "identifier": "c.typealias"
+      },
+      "location": {
+        "position": {
+          "character": 24,
+          "line": 19
+        },
+        "uri": "file://INPUT_DIR/input.h"
+      },
+      "names": {
+        "navigator": [
+          {
+            "kind": "identifier",
+            "spelling": "ExposedTypedefToHidden"
+          }
+        ],
+        "subHeading": [
+          {
+            "kind": "identifier",
+            "spelling": "ExposedTypedefToHidden"
+          }
+        ],
+        "title": "ExposedTypedefToHidden"
+      },
+      "pathComponents": [
+        "ExposedTypedefToHidden"
+      ],
+      "type": "c:@SA@_HiddenTypedef"
+    }
+  ]
+}