Background Weight stigma—a social phenomenon in which individuals can be treated differently due to their weight and physical appearance—can be associated with stress-related conditions and poor health. Sexual minority women (SMW) have an increased prevalence of obesity and overweight compared to heterosexual women, potentially making them more susceptible to experiencing weight stigma. Objectives No studies have examined weight stigma in a national sample of SMW. Therefore, the purpose of this study was to examine the prevalence of external weight stigma and internal weight stigma in a sample of SMW and in relevant subgroups defined by demographic data. Methods This study used online survey methods to determine the prevalence of external and internal weight stigma in a national sample of adult SMW ( n = 459). Results Most participants identified as non-Hispanic White, more than half identified as bisexual, and the sample ranged in age from 18 to 76 years. The prevalence of external weight stigma ranged from 26% to 65% depending on type (e.g., discrimination vs. mistreatment), and the prevalence of internal weight stigma was 57%. All body mass index categories were positively correlated with some forms of weight stigma, with the obese category associated with all forms of weight stigma. Discussion To address weight stigma, including eliminating discrimination based on weight and establishing caring and empathetic relationships, healthcare providers should support an individualized and holistic approach to weight management with attention to marginalized identities and other social determinants of health.