Edward Leung Tin-kei of Hong Kong Indigenous has admitted he was born in mainland China.
Leung has been critical of the government’s inability to control people from across the border from coming to Hong Kong.
His group has also staged protests against parallel traders from the mainland, accusing them of stealing the city’s resources.
But on Saturday, while he was out thanking voters in New Territories East, the localist told media he was born on the mainland.
He also revealed that his mother was a mainland immigrant who moved to the city 24 years ago.
The information has triggered heated debate on the internet as to whether Leung’s background is incompatible with his politics.
"Go away! You are only pretending to be a localist to gain votes," one internet user wrote.
"You were once a new immigrant, fighting for resources and school places with locals. Why do you now discriminate against new immigrants and mainlanders?" said another internet user.
Others, however, came to Leung’s defence: "It’s not about where you were born but where you grew up," one argued.
Speaking to the media on Saturday, Leung dismissed accusations that his place of birth jarred with his views.
The localist said what was important was that he defended Hong Kong values, culture and institutions, not where he was born.
And in response to his mother’s background, Leung stressed she had been living and working in the city for over two decades and could speak Cantonese.
Chung Kim-wah, a social scientist at the Polytechnic University, questioned whether Leung tried to hide his place of birth.
But Chung stressed that Leung’s place of birth was irrelevant in determining whether he was a local.
"If he came to Hong Kong when he was very young and he grew up here, then it’s natural he would feel attached to the city," Chung said.
The social scientist added that many in Hong Kong were not born locally but should still be considered local.
"As long as they have spent seven years here ... then they are a part of the city."