A staff member of the US Consulate General in Hong Kong was attacked by a man outside his workplace on Tuesday afternoon, police and a US official said.
The financial hub's police said they received a report on an attack in the afternoon which a consulate employee suffered a bloody injury to his head after a man struck him and fled.
A spokesperson of the consulate confirmed a staff member had been "assaulted by an unknown individual".
"The employee did not sustain life-threatening injuries. We cannot speculate on the assailant's motives at this time," the consulate official added.
Once a beacon of stability, Hong Kong has been left bitterly divided and polarised after last year's huge and sometimes violent pro-democracy protests.
Beijing has rejected calls for greater democracy and often blamed Washington for instigating and condoning the rallies.
It has responded with a crackdown on protesters, including the imposition of a sweeping new security law which has prompted the US to sanction some Chinese and Hong Kong officials.