The Born This Way hitmaker is working with the legendary singer on a new jazz-themed album and she stunned Bennett's fans by joining him on stage during his show at Montreal's Salle Wilfrid-Pelletier venue to showcase some of their material.
They performed a number of tracks including jazz standard Lush Life, which Gaga declared to be her "favourite song".
The pop star later shared pictures of herself with Bennett on her Instagram.com page, writing, "Surprise! I sang with Tony tonight in Montreal's Jazz Festival!... I had the time of my life tonight."
The pair worked together on Bennett's 2011 album Duets Ii and have remained close ever since. Their new record Cheek to Cheek is set to hit shelves later this year (14).