CCTV footage shows a man repeatedly punching the 63-year-old victim to the ground before hitting him on the head five times, leaving him unconscious.
The incident happened on the 149 bus just before it stopped at Seven Sisters station in the north London around 9.45 p.m. on August 23, the Metropolitan Police said.
Detectives say the victim, an NHS tracker, got on the bus through the middle doors in front of the suspect.
Five minutes later, the suspect followed the victim to the back doors, where police believe he had gone to try to create distance between them.
The suspect is believed to be “offended” when the victim walked away from him because he was not wearing his mask properly, the investigating officer said.
PC Bowman, Highway and Transportation Police Command, said: “Although there is no sound on the CCTV and all the victims remember waking up in the hospital, we believe everything it was triggered by face masks.
“We think the perpetrator, who had his face covered under his chin, took offense when the victim, who was fully covered, consciously walked away from him.
“When the perpetrator then follows the victim to the back of the bus, he can be seen pointing and gesturing towards the victim’s face mask before starting to punch and kick him several times.
“It was a completely unwarranted violent assault and we urgently need to speak to this man. “
The suspect is described as a tall black man, aged about 23 to 25, tall, with an athletic or muscular build.
He was wearing blue sweatpants, a white waistcoat and white sneakers.
Anyone with information should call detectives on 07880429486 or 101 quoting CAD reference 7981 / 23Aug. You can also call Crimestoppers anonymously on 0800 555 111 or tweet @MetCC with any information.