Weight stigma is the negative social judgment of people based on their weight. Despite the negative consequences of weight stigma, its prevalence is well-documented including within the physical activity (PA) landscape. Research shows that a wide range of PA-related professionals hold negative attitudes toward fat people, including physical education teachers, kinesiology students, and fitness trainers. This article describes a collaboration across disciplines and fields to develop a weight stigma reduction intervention for exercise professionals. Weight Inclusive Thinking for Fitness Spaces (WIT FITS) is a two-hour, self-paced, interactive online course grounded in the Health at Every Size® paradigm and research-informed strategies for improving exercise professionals' attitudes toward fatness and fostering weight inclusivity in fitness settings. WIT FITS was the result of a three-year interdisciplinary collaboration with scholars from pedagogy, public health, sport and exercise psychology, and nutrition. Community partners involved in the development, implementation, and evaluation of WIT FITS included body liberation activists, web and instructional designers, and campus recreation leaders.