Minaj, who recently confirmed that she was in negotiations for the US jaunt, agreed a deal "several days ago" with Spears's team, TMZ reports.
The 'Girls Fall Like Dominoes' rapper will replace Enrique Iglesias for the 26 live shows across the US, after he pulled out for unknown reasons last month.
In a post on his WhoSay page, he said: "Hey guys, sorry for the confusion regarding a possible tour in the summer with Britney Spears."
Spears's tour is scheduled to kick off on June 17 in Sacramento, California.
Watch Nicki Minaj featuring on Trey Songz's latest single 'Bottoms Up' below: