The deal was signed within the previous few days, with Minaj to join Spears on her 26-date run across America and Canada.
Minaj, a New York Hip Hop and R&B singer, was called in after Iglesias pulled out because his "ego could not handle opening for Britney," TMZ said.
Tour sources said Minaj would be an "infinitely better" support act than Iglesias, and that she was expected to generate more ticket sales.
Minaj currently has two singles on the Billboard Top 100 -- "Moment 4 Life" and "Did It On 'Em" -- and is on tour with Lil Wayne.
The "Femme Fatale" tour kicks off in Sacramento, Calif., on June 17 before winding around America and finishes with a show in Toronto, Canada, on Aug. 13.