Gender expression varies considerably among lesbian and bisexual women and may be related to various stressors and health outcomes. However, no current measure adequately assesses gender expression in this community. Thus, the authors conducted three studies to develop and validate the Gender Expression Measure among Sexual Minority Women (GEM-SMW). First, an initial item pool was evaluated in 3 focus groups with 23 women identifying as butch, femme, or neither. Second, responses from 459 lesbian and bisexual women who completed the revised items as part of an online survey were subject to confirmatory factor analysis (CFA), validating 15 items comprising 3 distinct factors assessing appearance, gender roles, and emotional expression. Third, the final model was replicated with a separate online sample of 1,381 lesbian and bisexual women, and measurement invariance (MI) testing demonstrated the stability of the GEM-SMW's factor structure. The GEM-SMW allows researchers to investigate gender expression without requiring participants to specify a gender identity (e.g., butch/femme). It is one of the first gender expression instruments developed specifically for lesbian and bisexual women, yet items may also be applicable for heterosexual women. Finally, the GEM-SMW provides opportunities to investigate the relation of gender expression with other constructs of interest such as identity development, stress, coping, and health. (Contains 7 tables.)