This study investigated patterns of sexual agreement for extra-dyadic sex and their associations with sexual risk behaviors among men who have sex with men (MSM) having a regular male sex partner (RP) in China. A cross-sectional telephone survey was conducted among 530 MSM recruited through multiple sources in Hong Kong, China, between April and December 2020. This study was based on a subsample of 368 participants who had an RP in the past 6 months. Logistic regression models were fitted. Among the participants, 27.2%, 13.0%, and 3.0% had a closed agreement, an in-between agreement, and an open agreement, respectively. Compared to no agreement, a closed agreement was associated with fewer extra-dyadic partners and fewer instances of condomless sex with extra-dyadic partners. Those who had more positive attitudes toward a closed agreement, perceived more support from significant others to create a closed agreement, and perceived higher behavioral control of refraining from sex with extra-dyadic partners were more likely to have a closed agreement with RP. Those who were concerned that a closed agreement would impair freedom and sexual desire were less likely to have such an agreement. A closed agreement is a potentially useful risk reduction strategy for Chinese MSM with an RP.