(1) Background: Obesity is associated with significant social consequences, and individuals with obesity are regularly affected by weight-related stigmatization experiences. This study compares antifat attitudes among registered dietitians (RD), nutrition students, and laypeople and assesses which factors related to the perceived causes of obesity influence these attitudes. (2) Methods: An online survey was conducted in Brazil with RD (n = 336), nutrition students (n = 300), and laypeople (n = 403) with questionnaires assessing antifat attitudes and perceived causes of obesity. (3) Results: All groups presented low antifat attitudes. Minor differences in antifat attitudes were found among the three groups. Compared to RDs and nutrition students, laypeople presented higher Weight Control/Blame scores, but with a small effect size (η2 = 0.01). Weight bias was predicted by age, sex, and body mass index. External, social, and financial factors were not perceived to be very important in the development of obesity by RD and students. (4) Conclusions: Since slight differences were seen among RD and students compared to laypeople, and some perceptions of the causes of obesity indicate a stigmatized view. It is essential to place a greater focus on educating and updating these health professionals and students about weight stigma and its consequences for the mental and physical health of individuals.