diff --git a/zuul/driver/github/githubconnection.py b/zuul/driver/github/githubconnection.py
index 218c82c93f..d34dfbf5be 100644
--- a/zuul/driver/github/githubconnection.py
+++ b/zuul/driver/github/githubconnection.py
@@ -71,6 +71,26 @@ class UTC(datetime.tzinfo):
 utc = UTC()
 
 
+class GithubShaCache(object):
+    def __init__(self):
+        self.projects = {}
+
+    def update(self, project_name, pr):
+        project_cache = self.projects.setdefault(project_name, {})
+        sha = pr['head']['sha']
+        number = pr['number']
+        cached_prs = project_cache.setdefault(sha, set())
+        if pr['state'] == 'open':
+            cached_prs.add(number)
+        else:
+            cached_prs.discard(number)
+
+    def get(self, project_name, sha):
+        project_cache = self.projects.get(project_name, {})
+        cached_prs = project_cache.get(sha, set())
+        return cached_prs
+
+
 class GithubGearmanWorker(object):
     """A thread that answers gearman requests"""
     log = logging.getLogger("zuul.GithubGearmanWorker")
@@ -514,6 +534,7 @@ class GithubConnection(BaseConnection):
             'canonical_hostname', self.server)
         self.source = driver.getSource(self)
         self.event_queue = queue.Queue()
+        self._sha_pr_cache = GithubShaCache()
 
         # Logging of rate limit is optional as this does additional requests
         rate_limit_logging = self.connection_config.get(
@@ -1125,6 +1146,9 @@ class GithubConnection(BaseConnection):
             pr['files'] = []
 
         pr['labels'] = [l.name for l in issueobj.labels()]
+
+        self._sha_pr_cache.update(project_name, pr)
+
         log.debug('Got PR %s#%s', project_name, number)
         self.log_rate_limit(self.log, github)
         return pr
@@ -1162,16 +1186,26 @@ class GithubConnection(BaseConnection):
         return True
 
     def getPullBySha(self, sha, project, log):
+        cached_pr_numbers = self._sha_pr_cache.get(project, sha)
+        if len(cached_pr_numbers) > 1:
+            raise Exception('Multiple pulls found with head sha %s' % sha)
+        if len(cached_pr_numbers) == 1:
+            for pr in cached_pr_numbers:
+                return self.getPull(project, pr, log)
+
         pulls = []
+        project_name = project
         owner, project = project.split('/')
         github = self.getGithubClient("%s/%s" % (owner, project))
         repo = github.repository(owner, project)
         for pr in repo.pull_requests(state='open'):
+            pr_dict = pr.as_dict()
+            self._sha_pr_cache.update(project_name, pr_dict)
             if pr.head.sha != sha:
                 continue
-            if pr.as_dict() in pulls:
+            if pr_dict in pulls:
                 continue
-            pulls.append(pr.as_dict())
+            pulls.append(pr_dict)
 
         log.debug('Got PR on project %s for sha %s', project, sha)
         self.log_rate_limit(self.log, github)