Claressa Shields beat Nikki Adler to win the IBF and WBC super-middleweight belts in her first title fight since turning professional.
The 22-year-old American won by a technical knockout after the fight was stopped in the fifth round in Detroit.
Shields is already a double Olympic champion after picking up gold medals at London 2012 and Rio 2016.
She turned professional in November after winning 77 fights and losing only one as an amateur.
Adler, from Germany, was previously undefeated in 16 fights but was undone by Shields in just her fourth pro fight.
In March, Shields contested the first women's boxing match to achieve main-event status on premium television in America.