Male partner involvement is an important factor in prevention of mother-to-child transmission (PMTCT). Yet, poor conceptualization has hindered optimal assessment of male involvement. We created and evaluated a brief 10-item male partner involvement scale using principal components analysis and scree plots, Cronbach’s alpha, and linear regression with survey data from postpartum women with HIV (n = 200) in Kenya. The scale had a two-factor structure: male encouragement/reminders and active participation. The overall scale and the encouragement/reminders sub-scale displayed strong internal reliability. In the multivariable models, the scales were positively associated with constructive relationship dynamics, HIV status disclosure, and couple HIV testing and counseling, and negatively associated with internalized HIV stigma. The encouragements/reminders sub-scale was also negatively associated with a new HIV diagnosis during pregnancy. This work furthers the conceptualization of male partner involvement in PMTCT and provides a valid measure to assess male involvement as a pathway to better PMTCT outcomes.