Objective: Elevated physical activity has been observed in some patients with anorexia nervosa (AN) despite their emaciated condition. However, its effects on treatment outcome remain unclear. This study aimed to examine objectively measured physical activity in this clinical population and how it might be related to a partial hospitalization therapy response, after considering potential confounders. Method: The sample comprised 88 AN patients consecutively enrolled in a day hospital treatment program, and 116 healthy-weight controls. All participants were female and a baseline assessment took place using an accelerometer (Actiwatch AW7) to measure physical activity, the Eating Disorders Inventory-2 and the Depression subscale of the Symptom Checklist-Revised. Outcome was evaluated upon the termination of the treatment program by expert clinicians. Results: Although AN patients and controls did not differ in the average time spent in moderate-to-vigorous physical activity (MVPA) (P = .21), nor daytime physical activity (P = .34), fewer AN patients presented a high physical activity profile compared to the controls (37% vs. 61%, respectively; P = .014). Both lower levels of MVPA and greater eating disorder severity had a direct effect on a poor treatment outcome. Depression symptoms in the patients were associated with lower MVPA, as well as with an older age, a shorter duration of the disorder and greater eating disorder psychopathology. Conclusions: There is a notable variation in the physical activity profile of AN patients, characterized by either low or very high patterns. Physical activity is a highly relevant issue in AN that must be taken into account during the treatment process.