Riley Marie Gaines Barker (née Gaines; born April 21, 2000) is an American conservative political activist and former collegiate swimmer known for campaigning against the participation of trans women in women's sports. A twelve-time NCAA All-American, she competed for the Kentucky Wildcats swim team in the Southeastern Conference before graduating and focusing on political activism.

Since 2023, Gaines has hosted the weekly OutKick and Fox Nation podcast Gaines for Girls.

Gaines was born and raised in Nashville, Tennessee. Her parents were active in sports. Her father, Brad Gaines, played football at Vanderbilt University and her mother, Telisha Gaines, played softball at Donelson Christian Academy and Austin Peay State University. Gaines attended Station Camp High School in Gallatin, Tennessee. As a junior, she won the 100-yard butterfly and 100-yard freestyle in the TISCA High School Swim & Dive Championship in Knoxville in 2017. She was invited to a 2016 United States Olympic Trials event, but failed to qualify after placing 85th.

Gaines joined the University of Kentucky's swim team and made the All-SEC Freshman Team in 2019. She also made the All-SEC Second Team in 2019 and 2020. She participated in the 2021 NCAA Women's Swimming & Diving Championships, coming seventh in the 200 freestyle race and winning a silver medal in the 4 × 200 yd freestyle relay; she made the All-SEC First Team that year. She was the 2022 SEC Women's Swimming and Diving Scholar-Athlete of the Year.

Gaines' highest-ranking individual event finish nationally was 5th place in the NCAA WD1 Championship 200yds Freestyle Final in March 2022. She did not pursue a professional career after graduating from college in 2022.

In March 2022, while swimming for the University of Kentucky in the 200-yard freestyle at the NCAA championship (her final competitive event race), Gaines tied for fifth place with University of Pennsylvania swimmer Lia Thomas, who subsequently became the first openly trans woman champion in the NCAA women's division after winning the 500-yard freestyle later at the same meet. While Thomas held the fifth-place trophy after the finish, Gaines held an equally-sized sixth-place trophy while waiting for another fifth-place trophy in the mail. Immediately after the meet, Gaines said in an interview with The Daily Wire: "I am in full support of her and full support of her transition and her swimming career...because there's no doubt that she works hard too, but she's just abiding by the rules that the NCAA put in place, and that's the issue."

In 2023, Gaines said that Thomas shared locker room space with her while still possessing "male genitalia". This incident began her activism.

Gaines has advocated against the inclusion of transgender women in the women's division of sports, including by lobbying her state representatives in April 2022 to pass a law that would prohibit transgender women from participating in women's sports.