Flames briefly flared around the man's head and neck as his helmet and plastic rain poncho caught fire during the incident on Sunday, Reuters pictures showed.
The journalist was burned on a "small part of his face" and was in hospital, a spokeswoman for Radio Television Hong Kong (RTHK), the territory's public broadcaster, said.
"Now he is fine," she said, declining to give the reporter's name, but confirming that he was from Hong Kong.
People nearby ran to help the man, who was hit by the device as he ran down a street where protests were happening, as seen in footage obtained by Reuters. A witness told Reuters that the device had been thrown by protesters.
The media worker, who was wearing a helmet marked Press, and a high-visibility yellow vest, had been involved in a live broadcast at the time, according to the spokeswoman.The incident occurred during the latest unrest in Hong Kong, with police firing tear gas and baton-charging crowds, while some demonstrators threw bricks and petrol bombs at police.