"Brad Beckworth, friend and advisor to Manziel and his family, has confirmed that Johnny entered treatment on Wednesday," the Browns announced in a team release.
"Johnny knows there are areas in which he needs to improve in order to be a better family member, friend and teammate and he thought the off-season was the right time to take this step," Beckworth said. "On behalf of Johnny and his family, we're asking for privacy until he rejoins the team in Cleveland."
From Browns general manager Ray Farmer: "We respect Johnny's initiative in this decision and will fully support him throughout this process. Our players' health and well-being will always be of the utmost importance to the Cleveland Browns. We continually strive to create a supportive environment and provide the appropriate resources, with our foremost focus being on the individual and not just the football player. Johnny's privacy will be respected by us during this very important period and we hope that others will do the same."
Manziel's off-field issues have been highly publicized since he was drafted in the first round in 2014.
The 22-year-old spent last season mostly as a backup quarterback behind Brian Hoyer. Manziel started two games, completing 18 of 35 passes (51.4 percent) for 175 yards, two interceptions and zero touchdowns.
The Browns recently hired new offensive coordinator John DeFilippo and quarterback coach Kevin O'Connell to work with Manziel on the field.
Manziel has taken the beginning step to right himself off the field first.