Dashawn McGrier is currently in the hospital being treated for his injuries, while his family fights for justice.
The viral video is now a part of an ongoing investigation that will also include reviewing body camera footage worn by the officer.
Interim police commissioner Gary Tuggle says he's deeply disturbed by the video and has accepted the officer's resignation a day after the officer was suspended.
"The officer involved in yesterday's incident is no longer with the BPD," the department said in a statement. "Interim Commissioner Gary Tuggle has accepted his resignation. The second officer remains on administrative duties. This remains an active criminal investigation."
Police said the second officer in the video didn't attempt to stop the other officer from throwing punches.
"I was angry. I was crying. I was hurt," McGrier's sister, Shantel Allen, said.
Investigators say the dispute started after one of the officers, who knew McGrier, asked him for an ID and he refused.
"The level of force that was used by that officer was unnecessary," Allen said.
My statement on the video of the Baltimore Police Officer pic.twitter.com/Wp5GJk80zt— Mayor Catherine Pugh (@MayorPugh50) August 11, 2018
Baltimore's mayor, Catherine Pugh, issued a statement demanding answers and accountability. She says the video was very disturbing.
Police say McGrier was released from custody and will not face any charges.
The officer, whose name has not been released, has worked for the Baltimore Police Department for just over a year, police said.