Rihanna's goin' on tour this summer -- and she's bringing Baby Gaga with her.

Rihanna & Ke$ha
PEOPLE has confirmed "Tik Tok" singer Ke$ha and the unbeweaveable Nicki Minaj will open for RiRi for her Last Girl On Earth tour.

The 25-date tour starts July 2 in Seattle. Tickets go on sale April 9.