Gaga welcomed her co-star and director on stage for a surprise duet during her latest "Enigma" concert at the Park Theater inside Park MGM. Cooper shyly walked onto the stage with a backward baseball cap, which he took off as Gaga mic'd him up.
"Hello, Vegas," Cooper said as he settled in next to Gaga, who played piano for the duet.
The song, which is featured in the film they starred in together, received an Oscar nomination for best original song.
This marked the first time the duo has performed the song together live.