Sexual agreements are ubiquitous among gay men. Lower levels of investment in these agreements may be associated with breaking them or engaging in risky sexual behavior. A scale was developed to measure agreement investment levels among gay men. Qualitative data from 78 gay men in committed relationships were analyzed to inform item development, followed by quantitative analyses of two larger samples (n = 380, n = 1,001) to assess construct, convergent, and discriminant validity. The Sexual Agreement Investment Scale (SAIS) is a psychometrically sound measure of the level of investment in sexual agreements among gay men in relationships. Men with higher agreement investment were less likely to break agreements and less likely to engage in unprotected anal intercourse with outside partners. The SAIS can be used to measure investment in sexual agreements and its impact on sexual behavior in a wide variety of settings, including research on relationships, sexuality, couples therapy and HIV prevention.