Background: Extreme dieting is a well-known phenomenon in combat sports, and still, little research has explored the link between extreme dieting and confidence levels among martial artists. Methods: In this cross-sectional study, extreme dieting and sport-specific self-confidence among 111 Swedish athletes practicing mixed martial arts (MMA) or Brazilian jiu-jitsu (BJJ) were examined. Athletes completed an online survey containing the Eating Disorder Examination questionnaire (EDE-Q) and the Trait Sport-Confidence inventory (TSCI). Results: The results showed that MMA athletes dieted in more extreme ways than BJJ athletes, primarily via restricted eating. They also had higher sport-specific self-confidence, which was positively correlated with weight loss. BJJ athletes used less restrictive eating than MMA athletes, but those who did diet in extreme ways experienced lower self-confidence compared to MMA athletes. Conclusions: The results are consistent with previous studies showing rapid weight loss in MMA athletes and suggest that some martial artists are at a particular risk of extreme dieting and possible sequelae. There is a link between self-confidence and weight loss, but it seems to allude to a comprehensive explanation and is in need of further research.