Michael Signora, the National Football League's senior vice president of football and international communications, announced the news in a tweet Friday.
Garrett's suspension will be at minimum for the remainder of the regular season and the postseason and the defensive end "must meet with the commissioner's office prior to a decision on his reinstatement," the statement read.
With just seconds left in Thursday’s game, Garrett snatched the helmet off Pittsburgh’s Mason Rudolph and smacked him in the head with it. The attack sparked a brawl involving multiple players from both teams.
Garrett and two other players — Cleveland defensive tackle Larry Ogunjobi and Pittsburgh center Maurkice Pouncey — were ejected from the game. Pouncey appeared to throw at least two punches and to kick Garrett in the helmet while he was on the ground.
Signora said in his statement that Ogunjobi and Pouncey were also suspended without pay “for their actions in Thursday night’s game.” Pouncey won't be able to play for three games and Ogunjobi for one game.
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Suspensions can be appealed within three business days.
Garrett apologized and said he made a "terrible mistake" in a statement released by the Browns on Friday.
"I lost my cool and what I did was selfish and unacceptable. I know that we are all responsible for our actions and I can only prove my true character through my actions moving forward," he said offering an apology to Rudolph, his teammates, the NFL and to his fans.
"I know I have to be accountable for what happened, learn from my mistake and I fully intend to do so," he said.
The team owners, Jimmy and Dee Haslam, said in a statement that they were "extremely disappointed" in Thursday night's attack and it "is not reflective of the core values we strive for as an organization."
The Steelers and the Browns have also each been fined $250,000 over the altercation. The three players have in addition been fined individually — Pouncey for punching and kicking a player, and Ogunjobi for "unnecessary roughness, specifically for shoving an opposing player to the ground during an altercation," the statement read.
The NFL official said Garrett "violated unnecessary roughness and unsportsmanlike conduct rules, as well as fighting, removing the helmet of an opponent and using the helmet as a weapon."
Browns quarterback Baker Mayfield on Thursday night condemned Garrett's actions as "inexcusable."
"I don't care, rivalry or not. We can't do that," he told Fox Sports following the game. The Browns won the game, played in Cleveland, 21-7.
"The reality is he's going to get suspended, and that's going to hurt our team," Mayfield added.
Freddie Kitchens, coach for the Browns, said the incident was embarrassing. "I'm embarrassed. Myles is embarrassed," he told reporters Thursday night. "It's not good."
Other players, including those who left the bench to go out onto the field where the fight was happening, will also be disciplined, Signora said.
Minyvonne Burke is a breaking news reporter for NBC News.