Background/Objectives: Fasting, orthorexia nervosa, and eating disorders are increasingly prevalent and interconnected. Understanding their relationship is essential for identifying potential risks and developing effective prevention and intervention strategies. This study investigated these associations to enhance our knowledge of their interplay and implications for mental health. Methods: A cross-sectional online survey was conducted in Poland in 2023. A sample of 214 participants aged 16 to 65 (M = 27.95, SD = 9.44) participated in this study. Fasting behavior was the predictor (independent) variable, orthorexia nervosa (measured using the Authorized Bratman Orthorexia Self-Test) was the mediator, and an eating disorder was the dependent variable (assessed using the Eating Attitude Test). Results: The Mann–Whitney U-test indicated that the fasting group scored higher in orthorexia and eating disorder symptoms than the non-fasting sample. Positive associations emerged between the fasting, orthorexia, and eating disorder scales. Linear regression analysis identified significant predictors of eating disorder symptoms, such as age, fasting, dieting, overweight status, and orthorexia. A path analysis revealed that fasting affected eating disorders directly and indirectly through orthorexia. Conclusions: This study identified fasting as a risk factor for orthorexia and other eating disorders, with orthorexia fully mediating the fasting–eating disorder relationship. Clinicians should consider both fasting and orthorexia when assessing patients at risk for eating disorders. This paper also proposes possible intervention and treatment strategies for affected individuals.