Police are trying to find and arrest him.
In the video, the shirtless attacker can be heard saying, "I'm tired of you asking for change," as he's attacking the man.
Several passengers told Channel 2 that the man being attacked is someone who rides the trains frequently, asking for money.
The incident happened just before 8:15 p.m. as the train was traveling east to the Georgia State station, MARTA police said in a Facebook post. Before the incident, the man was spotted at the Georgia State, Ashby and Oakland City stations.
MARTA sent the news station a statement regarding the incident that said: "We take matters of this nature seriously and request the ongoing assistance of our passengers in keeping our buses, trains and stations safe. If you witness violence or other criminal activity, please alert your bus or rail operator."
Police said the attacker will likely face assault charges when caught.