Pro-Beijing lawmaker Junius Ho is one of the most controversial politicians in Hong Kong, and has been heavily criticised by pro-democracy protesters.
Mr Ho was campaigning for an upcoming election when he was attacked by a man who appeared to pose as a supporter. The attacker was arrested shortly afterwards.
Video of the incident surfaced on social media and has been widely broadcast by Hong Kong news outlets.
Mr Ho and two other people were sent to hospital with injuries. He later posted a message online saying he was fine and that he had suffered a superficial wound.