Mikaela Mayer outpointed Mary Spencer in Canada to become a unified light-middleweight and a three-division world champion.
The 35-year-old American produced a dominant performance at the Montreal Casino, outboxing the hometown favourite over 10 rounds.
Judges scored the contest 100-90, 98-92 and 98-92 as Mayer claimed the WBA crown and the vacant WBC and WBO belts following trailblazer Cecilia Braekhus' retirement.
"I've won a WBO before but I've never had the WBA or WBC put around my waist, so this is extra special," Mayer said.
Mayer, who is also the reigning WBO champion at welterweight, won her first world title at super-featherweight. Her record now stands at 22 wins and two defeats.
The Californian quickly settled into her rhythm behind a sharp jab, varying her attacks to head and body.
Despite giving up two inches in height to the 5ft 11in Spencer, she controlled the range and tempo throughout.
Spencer, 40, enjoyed brief success in the fourth round when a pair of right hands connected, but Mayer reasserted herself, closing the eighth with four powerful hooks and a wide points victory looked inevitable.
Spencer was bloodied and marked under both eyes at the final bell as the Canadian suffered a third defeat in 13 bouts.
"She's a strong, solid opponent. Props to her, she didn't have to take this fight," Mayer said. "She took a big risk taking on a fighter like me. I always come forward and put on tough fights.
