Cleveland Browns quarterback
Manziel had been undergoing treatment for an undisclosed issue since Jan. 28. ESPN first reported Manziel's release.
Browns General Manager Ray Farmer expressed confidence last month that Manziel will challenge for the team's starting quarterback job when training camp starts. Making sure Manziel is healthy is the team's top priority, he said.
"We're definitely focused on his health and his well-being and we definitely want to support him in every way possible," Farmer said. "Part two of that is that inevitably he's going to have to continue to perform and show development and growth as a player. ... We all want to focus the opportunity on now."
Manziel has been criticized for his partying lifestyle reputation. In the past, he has posted photos of himself on Twitter and Instagram drinking at parties.
A star quarterback at Texas A&M who was drafted 22nd overall by the Browns in 2014, Manziel appeared in five games for the Browns last season, passing for 175 yards.