The sexual partnerships of transmasculine adults-who were assigned female at birth and identify on the masculine gender continuum-remain understudied. This includes characteristics of transmasculine adults' sexual partnerships associated with engaging in HIV/sexually transmitted infection (STI) sexual risk behavior. This study examined individual- and partnership-level factors of transmasculine adults' sexual partnerships associated with using a protective barrier during sexual activity. Data came from cross-sectional surveys administered to 141 transmasculine adults. Participants provided demographic and sexual health information for up to three sexual partners from the past 12 months ( = 259 partnerships). Generalized estimating equations (GEEs) were used to investigate individual- and partnership-level factors associated with any use of a protective barrier during five sexual behaviors. Transmasculine participants engaged in an array of sexual behaviors with diverse sexual partners. Individual- and partnership-level factors of transmasculine adults' sexual partnerships were associated with their protective barrier use; however, these associations varied in statistical significance across the five sexual behaviors. At the individual level, younger participants had lower odds of protective barrier use during fingering or fisting. At the partnership level, protective barrier use was associated with a sexual partnership's configuration and the gender identity of a sexual partner. Relative to participants with cisgender female partners, those with cisgender male partners generally had lower odds of using a protective barrier. Study findings highlight the importance of studying factors associated with HIV/STI risk behavior located beyond the individual. These findings may have implications for improving measurements of HIV/STI-related risk for transmasculine adults.