Mary Cecilia Cain (born May 3, 1996) is an American professional middle distance runner from Bronxville, New York.

[5] She ran the 800 meters at the 2012 United States Olympic Trials and placed 18th, and also ran at the 2012 World Junior Championships in Athletics, where she broke the American high school girl's outdoor record in the 1500 m with a 4:11.01, surpassing the record of Jordan Hasay in the 2008 United States Olympic Trials by over 3 seconds.

[7] On January 16, 2013, Cain won the 3000 meter race at the University of Washington Indoor Preview meet, with what should be a record-setting time of 9:02.10 (high school),[8] superior to the 9:08.6 Outdoor record of Lynn Bjorklund set in 1975.

On November 20, 2013 it was announced that Cain was forgoing a college career in order to run professionally with the Nike Oregon Project.

[18] On January 16, 2014 Cain set a new World Junior Indoor Record in the 1000 meters, running 2:39.25 at the Boston University Multi-Team Meet.

[21] On February 8, Cain returned to Boston, across town at the Reggie Lewis Track and Athletic Center at the New Balance Grand Prix to put the doubt about the 1000 metres to rest.

[22] Cain earned a silver medal at the 2014 USA Outdoor Track and Field Championships in the 1500 meters in Sacramento, California on June 27 and 29 running 4:06.34 in the final.

[26] At the 2015 New York Millrose Games, Cain finished eighth in the women's Wanamaker Mile, a loss she attributed to "growing pains".

Now representing the Nike Oregon Project, Cain opened her outdoor season with a time of 4:12.62 in the 1500 meters at the 2016 Drake Relays.

[33][34] She earned a 1500 roster spot on Team USA at the 2016 NACAC Under-23 Championships in Athletics, and at the 2016 US Olympic Trials in Eugene, Oregon.

[35] In late 2019, the New York Times published a video op-ed featuring Cain, where she attributed her decline in running performance to adverse coaching in the Nike Oregon Project.

[36] In it, Cain claimed that the head coach, Alberto Salazar, and his assistants arbitrarily set for her an unreasonably low goal weight, 114 pounds (52 kg).

[36] Cain alleged that Salazar routinely shamed her into meeting this weight, and that this, combined with the training regimens, pushed her body into the syndrome RED-S, involving lost menses, for some three years, whereby she accrued five bone fractures.

[36] Subsequently, Sports Illustrated published nine other accounts by former Nike Oregon Project runners, going back to 2008, who confirmed Cain's claims of "an abusive, toxic culture under Salazar.

"[37] On October 11, 2021, Cain filed a $20 million lawsuit against Salazar and his employer, Nike, accusing them of inflicting emotional and physical abuse.