Objective: To investigate whether cognitive inflexibility could be identified using the Wisconsin Card Sorting Test (WCST) in patients with severe and extreme anorexia nervosa (AN) compared to healthy control participants (HCs). Method: We used the WCST to assess 34 patients with AN (mean age: 25.9 years, mean body mass index (BMI): 13.2 kg/m2) 3–7 days after admission to a specialized nutrition unit and 34 HCs. The Beck Depression Inventory II and the Eating Disorder Inventory 3 were distributed. Results: The patients displayed more perseveration than HCs controlled for age and years of education, with moderate effect sizes (perseverative responses (%): adjusted difference = −7.74, 95% CI: −14.29–(−1.20),p-value: 0.021; perseverative errors (%): adjusted difference = −6.01, 95% CI: −11.06–(−0.96),p-value: 0.020). There were no significant relationships between perseveration and depression, eating disorder symptoms, illness duration, or BMI. Discussion: Patients with severe and extreme AN demonstrated lower cognitive flexibility compared to HCs. Performance was not related to psychopathology or BMI. Patients with severe and extreme anorexia nervosa may not differ from less severe patients in cognitive flexibility performance. As this study exclusively focused on patients suffering from severe and extreme AN, potential correlations might be masked by a floor effect.