Police said Kevin Lau Chun-to, the former chief editor of Ming Pao, was stabbed three times as he was walking to his car, the New York Times reported.
Lau was listed in critical condition at a hospital. He sustained stab wounds to his back and legs.
Lau was visited in the hospital by Leung Chun-ying, Hong Kong's chief executive who is viewed as pro-Beijing and who was scrutinized by Ming Pao when Lau was its editor. Following his visit, Leung condemned "this savage act."
Lau's ouster at the newspaper prompted protests by journalists and others who expressed fear his departure was an example of the Chinese Communist Party's efforts to suppress independent media in the semi-autonomous Chinese territory, the Times said.
Francis Moriarty, head of the press freedom committee of the Foreign Correspondents Club of Hong Kong, said the "shocking" attack occurred three days after a protest over press freedom concerns in the territory.
"We urge the Hong Kong police to treat the incident with the utmost urgency, and to find and prosecute the culprits," the correspondents club said in a statement. "The growing number of attacks against members of the press in Hong Kong needs to be taken seriously by the local administration. Hong Kong's reputation as a free and international city will suffer if such crimes go unsolved and unpunished."
Hong Kong police said they had no suspects but were reviewing footage from security cameras at the scene. The attacker fled with an accomplice on a motorbike.