diff --git a/changelogs/unreleased/29034-fix-github-importer.yml b/changelogs/unreleased/29034-fix-github-importer.yml
new file mode 100644
index 00000000000..6d08db3d55d
--- /dev/null
+++ b/changelogs/unreleased/29034-fix-github-importer.yml
@@ -0,0 +1,4 @@
+---
+title: Fix name colision when importing GitHub pull requests from forked repositories
+merge_request: 9719
+author:
diff --git a/lib/gitlab/github_import/pull_request_formatter.rb b/lib/gitlab/github_import/pull_request_formatter.rb
index 4ea0200e89b..28812fd0cb9 100644
--- a/lib/gitlab/github_import/pull_request_formatter.rb
+++ b/lib/gitlab/github_import/pull_request_formatter.rb
@@ -38,7 +38,11 @@ module Gitlab
 
       def source_branch_name
         @source_branch_name ||= begin
-          source_branch_exists? ? source_branch_ref : "pull/#{number}/#{source_branch_ref}"
+          if cross_project?
+            "pull/#{number}/#{source_branch_repo.full_name}/#{source_branch_ref}"
+          else
+            source_branch_exists? ? source_branch_ref : "pull/#{number}/#{source_branch_ref}"
+          end
         end
       end
 
@@ -52,6 +56,10 @@ module Gitlab
         end
       end
 
+      def cross_project?
+        source_branch.repo.id != target_branch.repo.id
+      end
+
       private
 
       def state
diff --git a/spec/lib/gitlab/github_import/pull_request_formatter_spec.rb b/spec/lib/gitlab/github_import/pull_request_formatter_spec.rb
index eeef23a61c6..951cbea7857 100644
--- a/spec/lib/gitlab/github_import/pull_request_formatter_spec.rb
+++ b/spec/lib/gitlab/github_import/pull_request_formatter_spec.rb
@@ -8,9 +8,11 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
   let(:repository) { double(id: 1, fork: false) }
   let(:source_repo) { repository }
   let(:source_branch) { double(ref: 'branch-merged', repo: source_repo, sha: source_sha) }
+  let(:forked_source_repo) { double(id: 2, fork: true, name: 'otherproject', full_name: 'company/otherproject') }
   let(:target_repo) { repository }
   let(:target_branch) { double(ref: 'master', repo: target_repo, sha: target_sha) }
   let(:removed_branch) { double(ref: 'removed-branch', repo: source_repo, sha: '2e5d3239642f9161dcbbc4b70a211a68e5e45e2b') }
+  let(:forked_branch) { double(ref: 'master', repo: forked_source_repo, sha: '2e5d3239642f9161dcbbc4b70a211a68e5e45e2b') }
   let(:octocat) { double(id: 123456, login: 'octocat', email: 'octocat@example.com') }
   let(:created_at) { DateTime.strptime('2011-01-26T19:01:12Z') }
   let(:updated_at) { DateTime.strptime('2011-01-27T19:01:12Z') }
@@ -205,6 +207,14 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
         expect(pull_request.source_branch_name).to eq 'pull/1347/removed-branch'
       end
     end
+
+    context 'when source branch is from a fork' do
+      let(:raw_data) { double(base_data.merge(head: forked_branch)) }
+
+      it 'prefixes branch name with pull request number and project with namespace to avoid collision' do
+        expect(pull_request.source_branch_name).to eq 'pull/1347/company/otherproject/master'
+      end
+    end
   end
 
   shared_examples 'Gitlab::GithubImport::PullRequestFormatter#target_branch_name' do
@@ -271,6 +281,24 @@ describe Gitlab::GithubImport::PullRequestFormatter, lib: true do
     end
   end
 
+  describe '#cross_project?' do
+    context 'when source and target repositories are different' do
+      let(:raw_data) { double(base_data.merge(head: forked_branch)) }
+
+      it 'returns true' do
+        expect(pull_request.cross_project?).to eq true
+      end
+    end
+
+    context 'when source and target repositories are the same' do
+      let(:raw_data) { double(base_data.merge(head: source_branch)) }
+
+      it 'returns false' do
+        expect(pull_request.cross_project?).to eq false
+      end
+    end
+  end
+
   describe '#url' do
     let(:raw_data) { double(base_data) }