Legislator Junius Ho, his assistant and the alleged attacker were taken to hospital with unspecified injuries following the assault, police said.
The attacker's motive was unknown but Ho gained notoriety among anti-government protesters in July when he was filmed laughing and shaking hands with suspected triad gang members who assaulted peaceful demonstrators.
The attack on Ho comes amid more than five months of sometimes violent political unrest in the former British colony-turned semi-autonomous Chinese territory.
Further protests were planned on Wednesday at some of Hong Kong's universities, activists said. Police fired water cannon to disperse protesters at a Guy Fawkes-themed march on Tuesday.
China's Communist Party said on Tuesday it would not tolerate any "separatist behavior" in Hong Kong, after some of the protesters called for independence.
What started as a protest against a proposed China extradition bill has widened into the gravest challenge to Chinese President Xi Jinping's rule since he came to power in 2012.
Protesters are demanding an end to perceived Chinese meddling in the territory's affairs, as well as universal suffrage and an independent inquiry into alleged police brutality, among other demands.
Beijing denies interfering and blames foreign governments for fueling the unrest.
Xi met Hong Kong leader Carrie Lam on Tuesday in Shanghai, vouching support for her administration.
Following the meeting, Lam denied rumors that the government was considering an amnesty for protesters charged with offenses