The opt-out option has the advantages of potentially normalizing HIV testing and hence remove stigma involved in HIV testing among men who have sex with men (MSM). The present study investigated behavioral intention to use free routine opt-out HIV testing in primary care settings and associated factors among MSM in Hong Kong, China. A cross-sectional survey was conducted among 336 MSM recruited from multiple sources during October 2015 and September 2016. Of the participants, 37.2% intended to use free routine opt-out HIV testing in primary care settings in the next 12 months if it was made available. Adjusted for significant background variables, variables on positive attitudes, perceived subjective norm (i.e., perceived support from male partners for taking up routine opt-out testing) and perceived descriptive norm (i.e., perceived >60% of peers would use this option) were significantly associated with behavioral intention use such testing option. In addition, had ever tested for HIV, and perceived level of trust of health professionals in primary care settings were also significant. Routine opt-out HIV testing is a potential useful means to increase HIV testing coverage among MSM in Hong Kong. Policy makers should consider allocating resources to pilot and implement this testing option.