Lovato, 27, and Rousey, 33 were special guests on the new NBC show "Game On!" last Wednesday night.
Lovato was placed on Team Venus, which is led up by famed tennis pro Venus Williams, 39, and includes comedian Ian Karmel, 35.
Rousey was placed on Team Gronk, whose leader is NFL star Rob Gronkowski, 31, and includes comedian Bobby Lee, 48.
Throughout the episode, host Keegan-Michael Key, 49, took the teams through a series of funny physical challenges.