Objective Little is known about body composition in patients with psoriatic arthritis (PsA). Our objective was to compare body composition parameters in PsA patients and healthy controls and then investigate the effects of ustekinumab (UST) on body composition in patients with PsA. Methods At baseline, 30 PsA patients were compared cross-sectionally with 60 healthy controls without PsA, matched for age, sex, menopausal status, and body mass index (BMI). Thirty active PsA patients treated with UST were included in a 6-month open follow-up study. Body composition parameters were measured at baseline and 6 months of treatment. Results Body composition parameters were different in PsA patients compared to healthy controls; in PsA patients, total and appendicular lean mass were lower (P = 0.013 and P = 0.010, respectively), whereas total fat mass was higher (P < 0.001). In 30% of the PsA patients, skeletal muscle mass was below the cutoff for low muscle quantity (men 7.26 kg/m2, women 5.5 kg/m2), whereas no such change was observed in the control group. After 6 months of treatment with UST, there was no significant change in BMI in 18 of the PsA patients. Total lean mass decreased slightly (P = 0.046), whereas fat mass tended to increase, but not significantly. No significant changes in appendicular lean mass and skeletal muscle mass index were observed. Conclusion In this study, we found that PsA patients had higher fat mass and lower lean mass than healthy controls. At 6-months of treatment, total lean mass decreased slightly, whereas fat mass tended to increase, but not significantly.