Weight gain is a primary treatment goal for anorexia nervosa (AN); however little is known about heterogeneity in weight gain pattern during treatment. Preliminary evidence suggests weight gain trajectory is associated with treatment outcome. This study grouped patients using mixture modeling into weight gain trajectories, and compared predictors and treatment outcomes between trajectory groups. Women diagnosed with AN or subthreshold AN (N5211) completed self-report measures at admission and six-months after discharge from an integrated inpatient (IP)-partial hospitalization (PH) behavioral specialty eating disorders program. Gowned weights were measured daily. Three distinct trajectories emerged: negative quadratic (Optimal), negative quadratic with fast weight gain (Fast), and positive linear with slower weight gain (Slow). The majority of patients were assigned to the Optimal group. Trajectory groups differed on admission, discharge, and follow-up variables. The Fast group emerged as most distinct. Women in this group were more than twice as likely to binge and or vomit regularly compared with the other two groups and were most likely to achieve weight restoration by discharge and to have more positive weight outcomes at short-term follow-up. There were no group differences in eating disorder behavioral frequencies at follow-up when adjusting for behavioral severity at admission. Weight gain trajectory may serve as a personalized in-treatment marker of outcome and could inform research on moderators and mediators of treatment response. Randomized controlled treatment studies, utilizing weight gain trajectories to determine group membership, may help identify subgroups of patients with differential responses to treatment interventions.