diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a4262c3..5d36322 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -118,6 +118,13 @@ jobs:
         run: |
           set -euo pipefail
           python scripts/validate_markdown_tables.py
+
+      - name: Validate cross-references
+        run: |
+          set -euo pipefail
+
+          python scripts/validate_cross_references.py
+
       - name: Check internal Markdown links
         run: |
           set -euo pipefail
diff --git a/scripts/validate_cross_references.py b/scripts/validate_cross_references.py
new file mode 100644
index 0000000..60281f1
--- /dev/null
+++ b/scripts/validate_cross_references.py
@@ -0,0 +1,56 @@
+from __future__ import annotations
+
+import re
+import sys
+from pathlib import Path
+
+from _ci_utils import (
+    display_path,
+    git_ls_files,
+    read_text,
+)
+
+def main() -> int:
+    valid_reqs = set()
+    req_pattern = re.compile(r"APTS-[A-Z]{2}-[A-Z0-9]{3}")
+    
+    md_files = git_ls_files("*.md")
+    standard_md_files = [f for f in md_files if Path(f).parts[0] == "standard"]
+    
+    for md_file in standard_md_files:
+        content = read_text(md_file)
+        
+        # Match definitions like "## APTS-RP-001: Title"
+        defs = re.findall(r"^#{2,4}\s+(APTS-[A-Z]{2}-[A-Z0-9]{3}):", content, re.MULTILINE)
+        valid_reqs.update(defs)
+        
+        # Match table definitions like "| APTS-RP-001 |"
+        table_defs = re.findall(r"\|\s*(APTS-[A-Z]{2}-[A-Z0-9]{3})\s*\|", content)
+        valid_reqs.update(table_defs)
+
+    if not valid_reqs:
+        print("Error: No requirements found to validate against.", file=sys.stderr)
+        return 1
+
+    failed = False
+    for md_file in standard_md_files:
+        content = read_text(md_file)
+        lines = content.splitlines()
+        
+        for i, line in enumerate(lines):
+            refs = req_pattern.findall(line)
+            for ref in refs:
+                if ref not in valid_reqs:
+                    print(f"Error in {display_path(md_file)}:{i+1}")
+                    print(f"  Invalid cross-reference: {ref} is not a known requirement.")
+                    failed = True
+
+    if failed:
+        print("Cross-reference validation failed.", file=sys.stderr)
+        return 1
+    
+    print(f"Successfully validated cross-references against {len(valid_reqs)} known requirements.")
+    return 0
+
+if __name__ == "__main__":
+    raise SystemExit(main())