This article reports the development and validation of a measure of Irrational Food Beliefs (IFB), defined as cognitively distorted and unhealthy attitudes and beliefs pertaining to food, which is proposed to be a factor that undermines success at weight loss and maintenance. Studies 1 and 2 demonstrated that the Irrational Food Beliefs Scale (IFBS) contains ecologically valid items and is comprised of two primary factors (irrational and rational food beliefs) whose items are internally consistent. Study 3 established that irrational subscale scores were positively associated with first semester weight gain in college freshmen. In Study 4, irrational food belief scores related predictably to measures of recent weight gain, poor weight loss maintenance, and bulimic symptoms in another college sample. In addition, IFB scores were not unduly influenced by test-taking response sets, and they were positively associated with depression and phobic anxiety and negatively correlated with self-esteem and need for cognition. Study 5 revealed strong positive associations between irrational food beliefs and bulimic symptoms, as well as number of previous diets in an obese community sample. The role irrational food beliefs may play in poor weight loss outcomes and maintenance is discussed.