Weight bias internalization (WBI) is an understudied form of internalized stigma, particularly among treatment-seeking adults with overweight/obesity. The current study surveyed 13,996 adults currently engaged in weight management in the first multinational study of WBI. From May to July 2020, participants in six Western countries completed the Modified Weight Bias Internalization Scale (WBIS-M) and measures of weight change, health behaviors, psychosocial well-being, and health-related quality of life (HRQOL). Participants were majority white, female, middle-aged, and categorized as having overweight or obesity based on body mass index. Results showed higher mean WBIS-M scores among participants in the UK, Australia, and France than in Germany, the US, and Canada. Across all countries, and controlling for participant characteristics and experiences of weight stigma, WBIS-M scores were associated with greater weight gain in the past year. Participants with higher WBIS-M scores also reported poorer mental and physical HRQOL, less eating and physical activity self-efficacy, greater engagement in eating as a coping strategy, more avoidance of going to the gym, poorer body image, and greater perceived stress. Few interaction effects were found between experiences and internalization of weight stigma. Overall, the current findings support WBI as a robust correlate of adverse weight-related health indices across six Western countries. Prospective and experimental studies are needed to determine directionality and causality in the relationship between WBI and poor health outcomes.