Dan Garrett said in a social media post that he was refused entry on Thursday for “unspecified immigration reasons”.
He added that it was the first time in 20 years of visiting Hong Kong that he has been turned away.
A spokesman for the US Congressional-Executive Commission on China said Garrett was told he could visit Hong Kong again in the future, but not right now.
Garrett, a political scientist who says he has documented more than 600 demonstrations in Hong Kong since 2011, appeared before the commission along with prominent activists from the SAR including Demosisto leader Joshua Wong and singer Denise Ho.
He told the hearing that Beijing sees dissidents in the SAR as “mortal threats” to the country, and the Hong Kong people have been “disenfranchised and terrorised with real and rhetorical political violence.”
The Hong Kong government refuses to comment on individual immigration cases.