Videos of the incident have drawn angry reactions across the world, and show the man's face being smacked on an armrest so hard that it makes him bleed, before being dragged from his chair and through the aisle.
In one particularly disturbing video, the man can be heard muttering "please kill me" while his mouth is covered in blood.
According to witnesses, United Airlines needed to put some extra crew members on the plane so they could make it in time for another flight - but there were too many passengers already onboard, so United attempted to make some room for them.
Some people were willing to swap flights to make room for the extra crew - but not enough, even when they were offered US$800 to do so - and so the airline then took to picking people at random and removing them by force.
The man they chose was a doctor, who flat refused to get off the plane because he had patients he needed to see at work the following day - but police officers were brought in and forcibly removed him from his seat.
Fellow passengers can be heard complaining loudly, with one woman clearly upset by what she was witnessing. Another passenger posted footage of the incident on Facebook, saying she was "shaky and so disgusted".
Following the Twitter storm that followed the incident, United Airlines chief executive Oscar Munoz apologised for the "upsetting event", saying he was sorry that customers had to "re-accommodate".
The apology drew more derision from Twitter users, who said Mr Munoz had failed to apologise specifically to the passenger featured in the video - and now many have threatened to boycott United.