Obesity is thought to be involved in the pathogenesis of psoriasis, although its impact on the therapeutic response to systemic treatments remains unclear. The aim of this study was to examine the association of body mass index (BMI) with the efficacy of ustekinumab in Japanese patients with psoriasis. Clinical data from a cohort of 111 Japanese patients treated with ustekinumab 45 mg between July 2011 and March 2014 were retrospectively evaluated. The measured outcome was improvement in the psoriasis area and severity index (PASI) score at week 16. Patients with BMI ≥ 25 and BMI < 25 had comparable rates of ≥50 and 75 % improvement in PASI (PASI-50 and PASI-75, respectively), whereas patients with BMI ≥ 25 had significantly lower PASI-90 and PASI-100 response rates. Patients with BMI ≥ 25 also showed significantly lower percent reduction in PASI than those with BMI < 25 at week 16 (85 vs. 74 %, P < 0.004). BMI was negatively correlated with percent reduction in PASI, whereas body weight was not. These results show that a higher BMI, but not body weight, is associated with lower effectiveness of ustekinumab for psoriasis. BMI ≥ 25 could therefore be a negative predictor of achieving PASI-90 and PASI-100 in patients with psoriasis when starting ustekinumab.