The pair performed together Gomez's 2015 single "Hands to Myself." Swift had also brought out Australian singer Sivan to perform his song "My, My, My," during the Saturday concert.
Swift posted on Instagram Sunday a photo of herself and Gomez sharing the stage together while smiling.
"To the person I could call at any time of day, who has been there no matter what... you absolutely KILLED IT tonight and everyone was so excited to see you. I love you, and 60,000 people at the Rose Bowl loved you too," she said.
The pop star also posted a video of herself and Gomez performing "Hands to Myself" amongst screaming fans. "I mean, I could but why would I want to? @selenagomez," she captioned the clip.
Swift, on Sunday, also won big at the 2018 Billboard Music Awards where she took home trophies for Top Female Artist and Top Selling Album for Reputation.