The fight lasted just 48 seconds, with Nunes (14-4) stunning Rousey (12-2) by TKO to retain her belt in the main event of UFC 207 at T-Mobile Arena.
Rousey had not fought since a stunning loss to Holly Holm in November 2015.
Her $3 million purse, which ESPN.com confirmed with Nevada State Athletic Commission officials, ties the richest disclosed fight purse in mixed martial arts history. UFC lightweight champion Conor McGregor netted the same amount for a non-title welterweight bout against Nate Diaz at UFC 202 in August.
McGregor's most recent purse for a lightweight title fight in November in New York was not disclosed.
Nunes (14-4) will make a guaranteed purse of $100,000, plus an additional potential $100,000 win bonus.
UFC president Dana White publicly stated Nunes will also receive a pay-per-view bonus. As one of the sport's biggest stars, Rousey is expected to profit on PPV sales as well.
Rousey's previous two fight purses were not disclosed, as they took place in jurisdictions that do not disclose purse information. Her last disclosed purse was $180,000 for a headlining fight against Cat Zingano at UFC 184 in February 2015 at Staples Center in Los Angeles.
Dominick Cruz will make a guaranteed purse of $350,000 for his fight against Cody Garbrandt, who captured Cruz's bantamweight belt after a five-round unanimous decision. Garbrandt will make $200,000.