Restrictive eating is common and associated with negative psychological outcomes across the life span and eating disorder (ED) severity levels. Little is known about functional processes that maintain restriction, especially outside of narrow diagnostic categories (e.g., anorexia nervosa). Here, we extend research on operant four-function models (identifying automatic negative, automatic positive, social negative, and social positive reinforcement functions) that have previously been applied to nonsuicidal self-injury (NSSI), binge eating, and purging to restricting. We assessed restrictive eating functions in three samples: clinically heterogeneous adolescents (Study 1: N = 457), transdiagnostic adults (Study 2: N = 145), and adults with acute or recently weight-restored anorexia nervosa (Study 3: N = 45). Study 1 indicated the four-function model was a good fit for restricting (root mean square error of approximation [RMSEA] = .06, Tucker-Lewis index [TLI] = .88). This factor structure replicated in Study 2 (comparative fit index [CFI] = .97, RMSEA = .07, TLI = .97, standardized root mean square residual [SRMR] = .09). Unlike NSSI, binge eating, and purging, which have been found to primarily serve automatic negative reinforcement functions, all three present studies found automatic positive reinforcement was most highly endorsed (by up to 85% of participants). In Studies 1 and 3, automatic functions were associated with poorer emotion regulation (ps < .05). In Study 1, social functions were associated with less social support (ps < .001). Across studies, automatic functions were associated with greater restriction ps < .05). Functions varied slightly by ED diagnosis. Across ED presentation, severity, and developmental stage, restrictive eating may be largely maintained by automatic positive reinforcement, with some variability across presentations. Continued examination of restrictive eating functions will establish processes that maintain restriction, allowing more precise treatment targeting for these problematic behaviors. (PsycInfo Database Record (c) 2021 APA, all rights reserved).