diff --git a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
--- a/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
+++ b/src/applications/diffusion/engine/DiffusionCommitHookEngine.php
@@ -164,6 +164,16 @@
         $this->applyHeraldRefRules($ref_updates);
       }
 
+      try {
+        if (!$is_initial_import) {
+          $this->rejectOversizedFiles($content_updates);
+        }
+      } catch (DiffusionCommitHookRejectException $ex) {
+        // If we're rejecting oversized files, flag everything.
+        $this->rejectCode = PhabricatorRepositoryPushLog::REJECT_OVERSIZED;
+        throw $ex;
+      }
+
       try {
         if (!$is_initial_import) {
           $this->rejectEnormousChanges($content_updates);
@@ -1255,6 +1265,139 @@
     return $changesets;
   }
 
+  private function rejectOversizedFiles(array $content_updates) {
+    $repository = $this->getRepository();
+
+    // TODO: Allow repositories to be configured for a maximum filesize.
+    $limit = 0;
+
+    if (!$limit) {
+      return;
+    }
+
+    foreach ($content_updates as $update) {
+      $identifier = $update->getRefNew();
+
+      $sizes = $this->loadFileSizesForCommit($identifier);
+      foreach ($sizes as $path => $size) {
+        if ($size <= $limit) {
+          continue;
+        }
+
+        $message = pht(
+          'OVERSIZED FILE'.
+          "\n".
+          'This repository ("%s") is configured with a maximum individual '.
+          'file size limit, but you are pushing a change ("%s") which causes '.
+          'the size of a file ("%s") to exceed the limit. The commit makes '.
+          'the file %s bytes long, but the limit for this repository is '.
+          '%s bytes.',
+          $repository->getDisplayName(),
+          $identifier,
+          $path,
+          new PhutilNumber($size),
+          new PhutilNumber($limit));
+
+        throw new DiffusionCommitHookRejectException($message);
+      }
+    }
+  }
+
+  public function loadFileSizesForCommit($identifier) {
+    $repository = $this->getRepository();
+    $vcs = $repository->getVersionControlSystem();
+
+    $path_sizes = array();
+
+    switch ($vcs) {
+      case PhabricatorRepositoryType::REPOSITORY_TYPE_GIT:
+        list($paths_raw) = $repository->execxLocalCommand(
+          'diff-tree -z -r --no-commit-id %s --',
+          $identifier);
+
+        // With "-z" we get "<fields>\0<filename>\0" for each line. Group the
+        // delimited text into "<fields>, <filename>" pairs.
+        $paths_raw = trim($paths_raw, "\0");
+        $paths_raw = explode("\0", $paths_raw);
+        if (count($paths_raw) % 2) {
+          throw new Exception(
+            pht(
+              'Unexpected number of output lines from "git diff-tree" when '.
+              'processing commit ("%s"): got %s lines, expected an even '.
+              'number.',
+              $identifier,
+              phutil_count($paths_raw)));
+        }
+        $paths_raw = array_chunk($paths_raw, 2);
+
+        $paths = array();
+        foreach ($paths_raw as $path_raw) {
+          list($fields, $pathname) = $path_raw;
+          $fields = explode(' ', $fields);
+
+          // Fields are:
+          //
+          //    :100644 100644 aaaa bbbb M
+          //
+          // [0] Old file mode.
+          // [1] New file mode.
+          // [2] Old object hash.
+          // [3] New object hash.
+          // [4] Change mode.
+
+          $paths[] = array(
+            'path' => $pathname,
+            'newHash' => $fields[3],
+          );
+        }
+
+        if ($paths) {
+          $check_paths = array();
+          foreach ($paths as $path) {
+            if ($path['newHash'] === self::EMPTY_HASH) {
+              $path_sizes[$path['path']] = 0;
+              continue;
+            }
+            $check_paths[$path['newHash']][] = $path['path'];
+          }
+
+          if ($check_paths) {
+            $future = $repository->getLocalCommandFuture(
+              'cat-file --batch-check=%s',
+              '%(objectsize)')
+              ->write(implode("\n", array_keys($check_paths)));
+
+            list($sizes) = $future->resolvex();
+            $sizes = trim($sizes);
+            $sizes = phutil_split_lines($sizes, false);
+            if (count($sizes) !== count($check_paths)) {
+              throw new Exception(
+                pht(
+                  'Unexpected number of output lines from "git cat-file" when '.
+                  'processing commit ("%s"): got %s lines, expected %s.',
+                  $identifier,
+                  phutil_count($sizes),
+                  phutil_count($check_paths)));
+            }
+
+            foreach ($check_paths as $object_hash => $path_names) {
+              $object_size = (int)array_shift($sizes);
+              foreach ($path_names as $path_name) {
+                $path_sizes[$path_name] = $object_size;
+              }
+            }
+          }
+        }
+        break;
+      default:
+        throw new Exception(
+          pht(
+            'File size limits are not supported for this VCS.'));
+    }
+
+    return $path_sizes;
+  }
+
   public function loadCommitRefForCommit($identifier) {
     $repository = $this->getRepository();
     $vcs = $repository->getVersionControlSystem();
diff --git a/src/applications/repository/storage/PhabricatorRepositoryPushLog.php b/src/applications/repository/storage/PhabricatorRepositoryPushLog.php
--- a/src/applications/repository/storage/PhabricatorRepositoryPushLog.php
+++ b/src/applications/repository/storage/PhabricatorRepositoryPushLog.php
@@ -24,6 +24,7 @@
   const CHANGEFLAG_REWRITE = 8;
   const CHANGEFLAG_DANGEROUS = 16;
   const CHANGEFLAG_ENORMOUS = 32;
+  const CHANGEFLAG_OVERSIZED = 64;
 
   const REJECT_ACCEPT = 0;
   const REJECT_DANGEROUS = 1;
@@ -31,6 +32,7 @@
   const REJECT_EXTERNAL = 3;
   const REJECT_BROKEN = 4;
   const REJECT_ENORMOUS = 5;
+  const REJECT_OVERSIZED = 6;
 
   protected $repositoryPHID;
   protected $epoch;
@@ -63,6 +65,7 @@
       self::CHANGEFLAG_REWRITE => pht('Rewrite'),
       self::CHANGEFLAG_DANGEROUS => pht('Dangerous'),
       self::CHANGEFLAG_ENORMOUS => pht('Enormous'),
+      self::CHANGEFLAG_OVERSIZED => pht('Oversized'),
     );
   }
 
@@ -74,6 +77,7 @@
       self::REJECT_EXTERNAL => pht('Rejected: External Hook'),
       self::REJECT_BROKEN => pht('Rejected: Broken'),
       self::REJECT_ENORMOUS => pht('Rejected: Enormous'),
+      self::REJECT_OVERSIZED => pht('Rejected: Oversized File'),
     );
   }