diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 61e1ee8..ec8b0dd 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -126,6 +126,75 @@ jobs:
           DATATOOLS_TESS_STAGING: build/_tesseract/${{ matrix.platform }}
         run: pyinstaller build/datatools.spec --clean --noconfirm
 
+      # ---- macOS code signing + notarization (before DMG packaging) -
+      # Signs dist/DataTools.app with the Developer ID, notarizes it,
+      # and staples the ticket so Gatekeeper passes offline. Wrapped in
+      # a guard: if the cert secret is absent the step prints a warning
+      # and exits 0, so dry-run dispatches still produce an (unsigned)
+      # build. Secret names match build/README.md "Signing".
+      - name: Sign & notarize macOS app
+        if: matrix.os == 'macos-latest'
+        env:
+          CERT_P12_BASE64: ${{ secrets.MACOS_DEVELOPER_ID_CERT_P12_BASE64 }}
+          CERT_PASSWORD:   ${{ secrets.MACOS_DEVELOPER_ID_CERT_PASSWORD }}
+          NOTARY_APPLE_ID: ${{ secrets.MACOS_NOTARY_APPLE_ID }}
+          NOTARY_TEAM_ID:  ${{ secrets.MACOS_NOTARY_TEAM_ID }}
+          NOTARY_PASSWORD: ${{ secrets.MACOS_NOTARY_PASSWORD }}
+        run: |
+          set -euo pipefail
+          if [ -z "${CERT_P12_BASE64:-}" ]; then
+            echo "::warning::MACOS_DEVELOPER_ID_CERT_P12_BASE64 not set — shipping an UNSIGNED build (Gatekeeper will warn buyers)."
+            exit 0
+          fi
+
+          APP="dist/DataTools.app"
+
+          # 1. Import the Developer ID cert into an ephemeral keychain.
+          KEYCHAIN="$RUNNER_TEMP/build.keychain-db"
+          KEYCHAIN_PW="$(uuidgen)"
+          security create-keychain -p "$KEYCHAIN_PW" "$KEYCHAIN"
+          security set-keychain-settings -lut 3600 "$KEYCHAIN"
+          security unlock-keychain -p "$KEYCHAIN_PW" "$KEYCHAIN"
+          echo "$CERT_P12_BASE64" | base64 --decode > "$RUNNER_TEMP/cert.p12"
+          security import "$RUNNER_TEMP/cert.p12" -k "$KEYCHAIN" -P "$CERT_PASSWORD" \
+            -T /usr/bin/codesign
+          security set-key-partition-list -S apple-tool:,apple: -s -k "$KEYCHAIN_PW" "$KEYCHAIN" >/dev/null
+          # Make the ephemeral keychain searchable (preserve the login keychain).
+          security list-keychains -d user -s "$KEYCHAIN" \
+            $(security list-keychains -d user | sed 's/"//g')
+
+          IDENTITY="$(security find-identity -v -p codesigning "$KEYCHAIN" \
+            | grep 'Developer ID Application' | head -1 | awk -F'"' '{print $2}')"
+          if [ -z "$IDENTITY" ]; then
+            echo "::error::No 'Developer ID Application' identity found in the imported cert."
+            exit 1
+          fi
+          echo "Signing with: $IDENTITY"
+
+          # 2. Sign the bundle (hardened runtime + secure timestamp + entitlements).
+          #    --deep signs the nested dylibs/.so the PyInstaller bundle carries.
+          codesign --deep --force --options runtime --timestamp \
+            --entitlements build/macos/entitlements.plist \
+            --sign "$IDENTITY" "$APP"
+          codesign --verify --strict --verbose=2 "$APP"
+
+          # 3. Notarize the .app (notarytool needs a zip/dmg/pkg, not a bare .app),
+          #    then staple so Gatekeeper validates offline.
+          if [ -n "${NOTARY_APPLE_ID:-}" ]; then
+            ditto -c -k --keepParent "$APP" "$RUNNER_TEMP/DataTools.zip"
+            xcrun notarytool submit "$RUNNER_TEMP/DataTools.zip" \
+              --apple-id "$NOTARY_APPLE_ID" \
+              --team-id "$NOTARY_TEAM_ID" \
+              --password "$NOTARY_PASSWORD" \
+              --wait
+            xcrun stapler staple "$APP"
+            xcrun stapler validate "$APP"
+          else
+            echo "::warning::Notary credentials not set — app is signed but NOT notarized (Gatekeeper will still warn)."
+          fi
+
+          rm -f "$RUNNER_TEMP/cert.p12"
+
       # ---- Per-platform installer packaging ------------------------
 
       - name: Package macOS DMG (installer)
diff --git a/build/README.md b/build/README.md
index 314dfcc..d4cbb86 100644
--- a/build/README.md
+++ b/build/README.md
@@ -112,12 +112,15 @@ pyinstaller build/datatools.spec --clean --noconfirm
 
 ## Signing (Phase 2 — needs accounts/credentials)
 
-Both code-signing steps are intentionally not in CI yet because they
-require credentials the owner sets up first.
+**macOS signing + notarization is now wired into `build.yml`** (the
+"Sign & notarize macOS app" step, with `build/macos/entitlements.plist`).
+It is guarded: if `MACOS_DEVELOPER_ID_CERT_P12_BASE64` is absent the step
+warns and exits 0, so dry-run dispatches still produce an unsigned build.
+To activate it, just add the secrets below — no code change needed.
+**Windows** code-signing is still not wired (accepted v1 friction).
 
 **macOS** — Apple Developer Program enrollment ($99/yr). Once enrolled,
-add these GitHub Secrets and uncomment the `codesign` + `notarytool`
-steps in `build.yml`:
+add these GitHub Secrets to activate the signing step in `build.yml`:
 
 | Secret | Value |
 |---|---|
diff --git a/build/macos/entitlements.plist b/build/macos/entitlements.plist
new file mode 100644
index 0000000..d833907
--- /dev/null
+++ b/build/macos/entitlements.plist
@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
+<!--
+  Hardened-runtime entitlements for the notarized DataTools.app.
+
+  PyInstaller freezes a CPython interpreter that maps writable+executable
+  memory and loads many unsigned .so/.dylib modules at runtime. Without
+  these entitlements the hardened runtime kills the process on launch
+  (or notarization rejects the bundle). Keep this list minimal — the app
+  is a local-only Streamlit server, so no network-server/device/camera
+  entitlements are needed.
+-->
+<plist version="1.0">
+<dict>
+  <!-- CPython JIT-style writable/executable memory + ctypes trampolines -->
+  <key>com.apple.security.cs.allow-jit</key>
+  <true/>
+  <key>com.apple.security.cs.allow-unsigned-executable-memory</key>
+  <true/>
+  <!-- Load the bundled C-extension .so / .dylib modules (pandas, pdfplumber,
+       Pillow, the bundled Tesseract dylibs) that aren't Team-ID signed -->
+  <key>com.apple.security.cs.disable-library-validation</key>
+  <true/>
+  <!-- Launcher sets DATATOOLS_*/TESSDATA_PREFIX/PYTHON* before exec -->
+  <key>com.apple.security.cs.allow-dyld-environment-variables</key>
+  <true/>
+</dict>
+</plist>