Michael Sam, who made history when he was picked by the St Louis Rams, says he has become concerned for his mental health.
Michael Sam, the first openly gay player picked in the NFL draft, has revealed he is "stepping away" from football because of concerns about his mental health.
Sam, who made history when he was selected in the seventh round by the St Louis Rams, announced his decision on Twitter.
He wrote: "The last 12 months have been very difficult for me, to the point where I became concerned with my mental health.
"Because of this I am going to step away from the game at this time."
The defensive lineman thanked the Canadian Football League's Montreal Alouettes, with whom he signed a two-year contract in May, and added he hoped to return soon.
The Alouettes have confirmed Sam has left for "personal reasons" and that he has been added to the team's suspended list.
He did not feature in Montreal's home defeat to Edmonton on Thursday night, which the club said was due to a sore back.
Sam had made his CFL debut the previous week, failing to record a tackle in Montreal's loss to Ottawa.
But Sam was suspended in June after he left the Alouettes' training camp for unexplained personal reasons.
He then sat out their first five games of the season.
Although Sam was selected by the Rams, he did not make their roster for the 2015 season.
He then spent time with the Dallas Cowboys, but was released from their practice squad in October.
Sam was a college star for the University of Missouri, where he was named the Southeastern Conference's co-defensive player of the year in 2013.