The singer was initially expected to perform with Enrique Iglesias, but he said a deal was never finalized and some media reports contended he didn't want to be Spears's opening act.
TMZ cited unnamed sources as saying Nicki Minaj will be the act that accompanies Spears on tour.
Minaj, who is now on tour with Lil Wayne, is known for her hits "Moment 4 Life" and "Did It On'em."
Spears posted a message on her Web site Monday saying: "I'm announcing the on-sale date for my tour tomorrow and the AMAZING Femme Fatales who will be joining me on stage this summer."
Spears's "Femme Fatale" tour is to begin June 17 in Sacramento.