Mikaela Joslin Mayer (born July 4, 1990) is an American professional boxer who has held world championships in three weight classes and is the current unified world female super-welterweight champion since October 2025, holding the WBA, WBC and WBO titles. She has also held the WBO welterweight title since September 2024. Mayer is a former unified female super-featherweight world champion, having held the WBO and IBF titles simultaneously. She has also held the interim WBC female lightweight title. As an amateur, she won a bronze medal at the 2012 World Championships and competed for the U.S. at the 2016 Olympics.
In 2025, Mayer was named Sports Illustrated<nowiki/>'s Female Fighter of the Year.
Mayer competed internationally as part of Team USA in the women's 60 kg category at the 2016 Rio Summer Olympics. She defeated Jennifer Chieng in the Round of 16 before being eliminated in the quarterfinal by Russian Anastasia Belyakova, who took a majority decision.
After signing a promotional deal with Top Rank, Mayer made her professional debut on 5 August 2017, defeating Widnelly Figueroa by first-round knockout.
Mayer beat Ewa Brodnicka by unanimous decision to win the WBO female super-featherweight World title at the MGM Grand in Las Vegas on 31 October 2020. Her Polish opponent had been reigning champion but was stripped of the belt after failing to make the 130-pound weight limit.
On 5 November 2021, she took on IBF female super-featherweight World champion Maïva Hamadouche in a contest that saw both women's titles and the inaugural Ring female super-featherweight belt on the line. Mayer prevailed by unanimous decision.
Seeking to further unify the division, Mayer faced WBC and IBO champion Alycia Baumgardner in London, England, on 15 October 2022. She lost via split decision with judges John Latham and Steve Gray making Baumgardner the winner, both scoring the fight 96-95, while Terry OâÂÂConnor gave it to Mayer 97-93.
Mayer switched to lightweight for her next outing where she was scheduled to face Christina Linardatou on 15 April 2023, but the Greek boxer failed her pre-fight medical. Sweden's Lucy Wildheart stepped in as a last-minute replacement with Mayer winning by unanimous decision to became the interim WBC female lightweight World champion.
Another change in weight division saw Mayer challenge IBF female welterweight World champion Natasha Jonas in Liverpool, England, on 20 January 2024. After a closely fought contest, it was the English boxer who came out on top winning by split decision with judge Diana Drews Milano scoring the fight for Mayer 97-93 but Frank Lombardi (96-94) and Michael Alexander (96-95) giving it to Jonas.
Mayer was scheduled to challenge WBO welterweight champion Sandy Ryan in New York on September 27, 2024. She won the fight by majority decision. On the day of the fight, while leaving to the event, Ryan was hit by a tin of paint at her hotel. Two days after the fight, Ryan demanded an immediate rematch in light of the paint incident and alleged threats against her and her support team.
Mayer made the first defense of her WBO welterweight title in a rematch against Sandy Ryan at Fontainebleau in Las Vegas on 29 March 2025. She won by unanimous decision.
Mayer challenged WBA female super-welterweight champion Mary Spencer on October 30, 2025, at Casino de Montreal in Montreal, Canada. The vacant WBC and WBO female super-welterweight titles were also on the line. Mayer won by unanimous decision.
In March 2026, it was announced that Mayer signed with Most Valuable Promotions.
Between 2003 and 2005, Mayer played bass in heavy metal band Lia-Fail, which also included Nita Strauss.
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