diff --git a/.github/scripts/resolve-hermes.mts b/.github/scripts/resolve-hermes.mts
index 8d6289573593..bfd3446d1b30 100644
--- a/.github/scripts/resolve-hermes.mts
+++ b/.github/scripts/resolve-hermes.mts
@@ -125,8 +125,9 @@ async function downloadUpstreamHermesTarball(
  * Extracts an upstream Hermes tarball and recomposes the xcframework to include
  * the macOS slice, if needed.
  *
- * Upstream tarballs ship a universal xcframework (iOS, simulator, catalyst,
- * tvOS, visionOS) plus a standalone macosx/hermes.framework. This function
+ * As of RN 0.83, the Hermes binary product is `hermesvm` (HermesV1). Upstream
+ * tarballs ship a universal `hermesvm.xcframework` (iOS, simulator, catalyst,
+ * tvOS, visionOS) plus a standalone `macosx/hermesvm.framework`. This function
  * merges the standalone macOS framework into the universal xcframework using
  * `xcodebuild -create-xcframework`.
  *
@@ -147,7 +148,7 @@ async function recomposeHermesXcframework(
   await $`tar -xzf ${tarballPath} -C ${destroot} --strip-components=2`;
 
   const frameworksDir = path.join(destroot, 'Library', 'Frameworks');
-  const xcfwPath = path.join(frameworksDir, 'universal', 'hermes.xcframework');
+  const xcfwPath = path.join(frameworksDir, 'universal', 'hermesvm.xcframework');
 
   echo('Upstream tarball contents:');
   await $`ls -la ${frameworksDir}`;
@@ -167,16 +168,16 @@ async function recomposeHermesXcframework(
   }
 
   // Check for standalone macOS framework
-  const standaloneMacFw = path.join(frameworksDir, 'macosx', 'hermes.framework');
+  const standaloneMacFw = path.join(frameworksDir, 'macosx', 'hermesvm.framework');
   if (!fs.existsSync(standaloneMacFw)) {
-    echo('ERROR: Upstream tarball missing macosx/hermes.framework');
+    echo('ERROR: Upstream tarball missing macosx/hermesvm.framework');
     return false;
   }
 
   // Collect existing frameworks from inside the universal xcframework
   const frameworkArgs: string[] = [];
   for (const entry of xcfwContents) {
-    const fwPath = path.join(xcfwPath, entry, 'hermes.framework');
+    const fwPath = path.join(xcfwPath, entry, 'hermesvm.framework');
     if (fs.existsSync(fwPath) && fs.statSync(fwPath).isDirectory()) {
       echo(`Found slice: ${fwPath}`);
       frameworkArgs.push('-framework', fwPath);
@@ -188,7 +189,7 @@ async function recomposeHermesXcframework(
   frameworkArgs.push('-framework', standaloneMacFw);
 
   // Build new xcframework at a temp path (frameworks reference paths inside the old xcfw)
-  const xcfwNew = path.join(frameworksDir, 'universal', 'hermes-new.xcframework');
+  const xcfwNew = path.join(frameworksDir, 'universal', 'hermesvm-new.xcframework');
   const sliceCount = frameworkArgs.filter(f => f !== '-framework').length;
   echo(`Creating new universal xcframework with ${sliceCount} slices...`);
   await $`xcodebuild -create-xcframework ${frameworkArgs} -output ${xcfwNew} -allow-internal-distribution`;
diff --git a/.github/workflows/microsoft-pr.yml b/.github/workflows/microsoft-pr.yml
index ee3a01e5dd01..f408a1b0c547 100644
--- a/.github/workflows/microsoft-pr.yml
+++ b/.github/workflows/microsoft-pr.yml
@@ -3,7 +3,7 @@ name: PR
 on:
   pull_request:
     types: [opened, synchronize, edited]
-    branches: [ "main", "*-stable", "release/*" ]
+    branches: [ "main", "*-stable", "release/*", "0.83-merge" ]
 
 concurrency:
   # Ensure single build of a pull request. `main` should not be affected.
diff --git a/.github/workflows/microsoft-resolve-hermes.yml b/.github/workflows/microsoft-resolve-hermes.yml
index 78f590715ddc..780f52cadae9 100644
--- a/.github/workflows/microsoft-resolve-hermes.yml
+++ b/.github/workflows/microsoft-resolve-hermes.yml
@@ -79,7 +79,7 @@ jobs:
         id: cache
         uses: actions/cache/restore@v4
         with:
-          key: hermes-v1-${{ steps.resolve.outputs.hermes-commit }}-Debug
+          key: hermesv1-engine-${{ steps.resolve.outputs.hermes-commit }}-Debug
           path: hermes-destroot
 
       - name: Upload cached Hermes artifacts
@@ -118,6 +118,8 @@ jobs:
           ref: ${{ needs.resolve-hermes.outputs.hermes-commit }}
           path: hermes
 
+      # HermesV1's CMakeLists requires CMAKE_BUILD_TYPE explicitly. The upstream
+      # `build_host_hermesc` helper doesn't set it, so we invoke cmake directly here.
       - name: Build hermesc
         working-directory: hermes
         env:
@@ -125,8 +127,8 @@ jobs:
           JSI_PATH: ${{ github.workspace }}/hermes/API/jsi
           MAC_DEPLOYMENT_TARGET: '14.0'
         run: |
-          source $GITHUB_WORKSPACE/packages/react-native/sdks/hermes-engine/utils/build-apple-framework.sh
-          build_host_hermesc
+          cmake -S . -B build_host_hermesc -DJSI_DIR="$JSI_PATH" -DCMAKE_BUILD_TYPE=Release
+          cmake --build ./build_host_hermesc --target hermesc -j "$(sysctl -n hw.ncpu)"
 
       - name: Upload hermesc artifact
         uses: actions/upload-artifact@v4
@@ -249,7 +251,7 @@ jobs:
       - name: Save Hermes cache
         uses: actions/cache/save@v4
         with:
-          key: hermes-v1-${{ needs.resolve-hermes.outputs.hermes-commit }}-Debug
+          key: hermesv1-engine-${{ needs.resolve-hermes.outputs.hermes-commit }}-Debug
           path: hermes/destroot
 
       - name: Upload Hermes artifacts
diff --git a/packages/react-native/Package.swift b/packages/react-native/Package.swift
index f6eefe500acd..ba1d02178ef6 100644
--- a/packages/react-native/Package.swift
+++ b/packages/react-native/Package.swift
@@ -945,6 +945,11 @@ extension Target {
         .define("DEBUG", .when(configuration: .debug)),
         .define("NDEBUG", .when(configuration: .release)),
         .define("USE_HERMES", to: "1"),
+        // [macOS] The SPM build links against hermesvm.xcframework (HermesV1).
+        // Several headers (e.g. ReactCommon/hermes/inspector-modern/chrome/Registration.h)
+        // gate legacy inspector code on `!defined(HERMES_V1_ENABLED)`, matching what the
+        // CocoaPods path sets via cocoapods/utils.rb when RCT_HERMES_V1_ENABLED=1.
+        .define("HERMES_V1_ENABLED", to: "1"),
       ] + defines + cxxCommonHeaderPaths
 
     return .target(
diff --git a/packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm b/packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm
index 17945ce64e40..73068815def1 100644
--- a/packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm
+++ b/packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm
@@ -807,7 +807,7 @@ - (RCTPlatformView *)betterHitTest:(CGPoint)point withEvent:(UIEvent *)event //
 
   BOOL isPointInside = [self pointInside:point withEvent:event];
 
-  UIView *currentContainerView = self.currentContainerView;
+  RCTPlatformView *currentContainerView = self.currentContainerView; // [macOS]
 
   BOOL clipsToBounds = false;
 
@@ -1045,19 +1045,19 @@ - (BOOL)styleWouldClipOverflowInk
 // `blur` applied, we need to wrap it in a SwiftUI view to render the effect.
 // In this case, `effectiveContentView` will be the content view inside the
 // SwiftUI wrapper.
-- (UIView *)effectiveContentView
+- (RCTPlatformView *)effectiveContentView // [macOS]
 {
   if (!ReactNativeFeatureFlags::enableSwiftUIBasedFilters()) {
     return self;
   }
 
-  UIView *effectiveContentView = self;
+  RCTPlatformView *effectiveContentView = self; // [macOS]
 
   if (self.styleNeedsSwiftUIContainer) {
     if (_swiftUIWrapper == nullptr) {
       _swiftUIWrapper = [RCTSwiftUIContainerViewWrapper new];
-      UIView *swiftUIContentView = [[UIView alloc] init];
-      for (UIView *subview = nullptr in self.subviews) {
+      RCTPlatformView *swiftUIContentView = [[RCTPlatformView alloc] init]; // [macOS]
+      for (RCTPlatformView *subview = nullptr in self.subviews) { // [macOS]
         [swiftUIContentView addSubview:subview];
       }
       swiftUIContentView.clipsToBounds = self.clipsToBounds;
@@ -1074,8 +1074,8 @@ - (UIView *)effectiveContentView
     effectiveContentView = _swiftUIWrapper.contentView;
   } else {
     if (_swiftUIWrapper != nullptr) {
-      UIView *swiftUIContentView = _swiftUIWrapper.contentView;
-      for (UIView *subview = nullptr in swiftUIContentView.subviews) {
+      RCTPlatformView *swiftUIContentView = _swiftUIWrapper.contentView; // [macOS]
+      for (RCTPlatformView *subview = nullptr in swiftUIContentView.subviews) { // [macOS]
         [self addSubview:subview];
       }
       self.clipsToBounds = swiftUIContentView.clipsToBounds;
@@ -1096,7 +1096,7 @@ - (UIView *)effectiveContentView
 // the view and is not affected by clipping.
 - (RCTUIView *)currentContainerView // [macOS]
 {
-  UIView *effectiveContentView = self.effectiveContentView;
+  RCTPlatformView *effectiveContentView = self.effectiveContentView; // [macOS]
 
   if (_useCustomContainerView) {
     if (!_containerView) {
@@ -1357,7 +1357,7 @@ - (void)invalidateLayer
       if (primitive.type == FilterType::DropShadow) {
         if (_swiftUIWrapper != nullptr && std::holds_alternative<DropShadowParams>(primitive.parameters)) {
           const auto &dropShadowParams = std::get<DropShadowParams>(primitive.parameters);
-          UIColor *shadowColor = RCTUIColorFromSharedColor(dropShadowParams.color);
+          RCTUIColor *shadowColor = RCTUIColorFromSharedColor(dropShadowParams.color); // [macOS]
           [_swiftUIWrapper updateDropShadow:@(dropShadowParams.standardDeviation)
                                           x:@(dropShadowParams.offsetX)
                                           y:@(dropShadowParams.offsetY)
@@ -2409,7 +2409,7 @@ - (BOOL)styleNeedsSwiftUIContainer
   return NO;
 }
 
-- (void)transferVisualPropertiesFromView:(UIView *)sourceView toView:(UIView *)destinationView
+- (void)transferVisualPropertiesFromView:(RCTPlatformView *)sourceView toView:(RCTPlatformView *)destinationView // [macOS]
 {
   // shadow
   destinationView.layer.shadowColor = sourceView.layer.shadowColor;
@@ -2457,6 +2457,12 @@ - (void)transferVisualPropertiesFromView:(UIView *)sourceView toView:(UIView *)d
   }
 }
 
+#if !TARGET_OS_OSX // [macOS]
+// macOS provides its own implementations of these methods inside the
+// `#if TARGET_OS_OSX` block above (see ~line 1846 onwards), so the iOS
+// definitions must be guarded to avoid duplicate-declaration errors and
+// to keep AppKit's responder model intact (NSView uses
+// `acceptsFirstResponder` rather than `canBecomeFirstResponder`).
 - (BOOL)canBecomeFirstResponder
 {
   return YES;
@@ -2512,6 +2518,7 @@ - (BOOL)resignFirstResponder
 
   return YES;
 }
+#endif // [macOS]
 
 @end
 
diff --git a/packages/react-native/scripts/ios-prebuild/microsoft-hermes.js b/packages/react-native/scripts/ios-prebuild/microsoft-hermes.js
index 9bdeec72d8fc..f882d749cc05 100644
--- a/packages/react-native/scripts/ios-prebuild/microsoft-hermes.js
+++ b/packages/react-native/scripts/ios-prebuild/microsoft-hermes.js
@@ -56,6 +56,29 @@ function findMatchingHermesVersion(
   return null;
 }
 
+/**
+ * Reads the pinned HermesV1 tag from sdks/.hermesv1version.
+ *
+ * Returns a value like 'hermes-v250829098.0.2', which can be used as a git ref
+ * when checking out facebook/hermes for from-source builds. Returns null if the
+ * file is missing or empty.
+ */
+function hermesV1Tag() /*: ?string */ {
+  const tagFile = path.resolve(
+    __dirname,
+    '..',
+    '..',
+    'sdks',
+    '.hermesv1version',
+  );
+  try {
+    const tag = fs.readFileSync(tagFile, 'utf8').trim();
+    return tag.length > 0 ? tag : null;
+  } catch (_) {
+    return null;
+  }
+}
+
 /**
  * Finds the Hermes commit at the merge base with facebook/react-native.
  * Used on the main branch (1000.0.0) where no prebuilt artifacts exist.
@@ -64,11 +87,15 @@ function findMatchingHermesVersion(
  * the latest Hermes commit because Hermes and JSI don't always guarantee backwards compatibility.
  * Instead, we take the commit hash of Hermes at the time of the merge base with facebook/react-native.
  *
+ * Hermes ships HermesV1 on the `static_h` branch as of RN 0.83, and the macOS fork's
+ * SPM build path consumes hermesvm — so we resolve against `static_h`, not `main`.
+ *
  * This is the JavaScript equivalent of the Ruby `hermes_commit_at_merge_base`
  * in sdks/hermes-engine/hermes-utils.rb.
  */
 function hermesCommitAtMergeBase() /*: {| commit: string, timestamp: string |} */ {
   const HERMES_GITHUB_URL = 'https://github.com/facebook/hermes.git';
+  const HERMES_BRANCH = 'static_h';
 
   // Fetch upstream react-native
   macosLog('Fetching facebook/react-native to find merge base...');
@@ -110,21 +137,20 @@ function hermesCommitAtMergeBase() /*: {| commit: string, timestamp: string |} *
   const hermesGitDir = path.join(tmpDir, 'hermes.git');
 
   try {
-    // Explicitly use Hermes 'main' branch since the default branch changed to 'static_h' (Hermes V1)
     execSync(
-      `git clone -q --bare --filter=blob:none --single-branch --branch main ${HERMES_GITHUB_URL} "${hermesGitDir}"`,
+      `git clone -q --bare --filter=blob:none --single-branch --branch ${HERMES_BRANCH} ${HERMES_GITHUB_URL} "${hermesGitDir}"`,
       {stdio: 'pipe', timeout: 120000},
     );
 
-    // Find the Hermes commit at the time of the merge base on branch 'main'
+    // Find the Hermes commit at the time of the merge base on the HermesV1 branch
     const commit = execSync(
-      `git --git-dir="${hermesGitDir}" rev-list -1 --before="${timestamp}" refs/heads/main`,
+      `git --git-dir="${hermesGitDir}" rev-list -1 --before="${timestamp}" refs/heads/${HERMES_BRANCH}`,
       {encoding: 'utf8'},
     ).trim();
 
     if (!commit) {
       abort(
-        `[Hermes] Unable to find the Hermes commit hash at time ${timestamp} on branch 'main'.`,
+        `[Hermes] Unable to find the Hermes commit hash at time ${timestamp} on branch '${HERMES_BRANCH}'.`,
       );
     }
 
@@ -196,6 +222,7 @@ function abort(message /*: string */) {
 module.exports = {
   findMatchingHermesVersion,
   hermesCommitAtMergeBase,
+  hermesV1Tag,
   findVersionAtMergeBase,
   getLatestStableVersionFromNPM,
 };
diff --git a/packages/react-native/sdks/hermes-engine/utils/build-apple-framework.sh b/packages/react-native/sdks/hermes-engine/utils/build-apple-framework.sh
index dbb363bd6870..bc62858e6dd9 100755
--- a/packages/react-native/sdks/hermes-engine/utils/build-apple-framework.sh
+++ b/packages/react-native/sdks/hermes-engine/utils/build-apple-framework.sh
@@ -163,11 +163,17 @@ function build_apple_framework {
     mkdir -p destroot/include/hermes/cdp
     cp API/hermes/cdp/*.h destroot/include/hermes/cdp
 
-    mkdir -p destroot/include/hermes/inspector
-    cp API/hermes/inspector/*.h destroot/include/hermes/inspector
+    # [macOS] HermesV1 (static_h) drops the legacy inspector headers in favour of
+    # API/hermes/cdp/. Skip the copy when the source directory has no headers.
+    if compgen -G "API/hermes/inspector/*.h" > /dev/null; then
+      mkdir -p destroot/include/hermes/inspector
+      cp API/hermes/inspector/*.h destroot/include/hermes/inspector
+    fi
 
-    mkdir -p destroot/include/hermes/inspector/chrome
-    cp API/hermes/inspector/chrome/*.h destroot/include/hermes/inspector/chrome
+    if compgen -G "API/hermes/inspector/chrome/*.h" > /dev/null; then
+      mkdir -p destroot/include/hermes/inspector/chrome
+      cp API/hermes/inspector/chrome/*.h destroot/include/hermes/inspector/chrome
+    fi
 
     mkdir -p destroot/include/jsi
     cp "$JSI_PATH"/jsi/*.h destroot/include/jsi
@@ -193,11 +199,17 @@ function prepare_dest_root_for_ci {
   mkdir -p destroot/include/hermes/cdp
   cp API/hermes/cdp/*.h destroot/include/hermes/cdp
 
-  mkdir -p destroot/include/hermes/inspector
-  cp API/hermes/inspector/*.h destroot/include/hermes/inspector
+  # [macOS] HermesV1 (static_h) drops the legacy inspector headers in favour of
+  # API/hermes/cdp/. Skip the copy when the source directory has no headers.
+  if compgen -G "API/hermes/inspector/*.h" > /dev/null; then
+    mkdir -p destroot/include/hermes/inspector
+    cp API/hermes/inspector/*.h destroot/include/hermes/inspector
+  fi
 
-  mkdir -p destroot/include/hermes/inspector/chrome
-  cp API/hermes/inspector/chrome/*.h destroot/include/hermes/inspector/chrome
+  if compgen -G "API/hermes/inspector/chrome/*.h" > /dev/null; then
+    mkdir -p destroot/include/hermes/inspector/chrome
+    cp API/hermes/inspector/chrome/*.h destroot/include/hermes/inspector/chrome
+  fi
 
   mkdir -p destroot/include/jsi
   cp "$JSI_PATH"/jsi/*.h destroot/include/jsi