Objective: Anorexia nervosa (AN) has been associated with cognitive impairment. While re-nutrition is one of the main treatment targets, the effect on cognitive impairments is unclear. The aim of this review was to examine whether cognitive functions improve after weight gain in patients with AN. Method: A systematic review was performed following Preferred Reporting Items for Systematic Reviews and Meta-analyses statement guidelines (PROSPERO CRD42019081993). Literature searches were conducted May 20th, 2019 in PubMed, EMBASE, PsychINFO and Cochrane Library. Pairs of reviewers screened reports independently based on titles/abstracts (N = 6539) and full texts (N = 378). Furthermore, they assessed the quality of reports, including whether practice effects were accounted for. Results: Twenty-four longitudinal reports were included featuring 757 patients and 419 healthy controls. Six studies examined children and adolescents. Four out of four studies found processing speed to improve above and beyond what could be assigned to practice effects and three out of four studies found that cognitive flexibility was unaffected after weight gain in children and adolescents. Results from studies of adults were inconclusive. Discussion: The literature on cognitive change in patients with AN following weight gain is sparse. Preliminary conclusions can be made only for children and adolescents, where weight gain appeared to be associated with improved processing speed.