Recent studies indicate the presence of systemic inflammation in psoriatic patients, and this inflammatory status is significantly associated with a range of comorbidities. The aim of this study was to evaluate the clinical significance of novel inflammatory biomarkers, neutrophil-lymphocyte ratio (NLR), platelet-lymphocyte ratio (PLR) and mean platelet volume (MPV) in Japanese patients with plaque-type psoriasis (PsV) and psoriatic arthritis (PsA). One hundred and eighty-six patients with PsV and 50 patients with PsA treated with biologics, including infliximab, adalimumab and ustekinumab, were retrospectively analyzed before and after treatment. At baseline, NLR and PLR, as well as C-reactive protein (CRP), were significantly higher in PsA patients than those in PsV patients, and a significant correlation was found between NLR and PLR. In PsV patients, the NLR-high and PLR-high subgroups exhibited significantly higher Psoriasis Area and Severity Index scores compared with the NLR-low and PLR-low subgroups, respectively, and the NLR-high subgroup also showed higher CRP levels. MPV value was negatively associated with the presence of arthritis, but its association with inflammation was less clear than that of NLR or PLR. After treatment of the patients with biologics for up to 12 months, NLR and PLR decreased promptly in parallel with a decrease of CRP, irrespective of the type of biologics used. Altogether, these results indicate that both NLR and PLR may be useful markers to evaluate systemic inflammation in psoriatic patients. They may serve as simple, convenient and cost-effective biomarkers to monitor the disease course after systemic therapy.