<b><i>Background:</i></b> Sitagliptin, a dipeptidyl peptidase-4 (DPP-4) inhibitor licensed for the treatment of type 2 diabetes mellitus (T2DM), has been reported to improve psoriasis. <b><i>Objective:</i></b> We compared the effects of sitagliptin treatment, a DPP-4 inhibitor, in combination with narrow-band ultraviolet-B (NB-UVB) phototherapy compared to NB-UVB alone on psoriasis severity, quality of life, cardiovascular disease risk factors and immune parameters in people with moderate psoriasis without T2DM. <b><i>Methods:</i></b> In this 39-week, single-centre, randomised controlled trial, people were allocated randomly to receive sitagliptin for 24 weeks with NB-UVB or NB-UVB alone. The primary endpoint was the change in Psoriasis Area and Severity Index (PASI) from baseline to 24 weeks. We estimated that 120 participants would be needed to have 80% power to find a significant difference between the groups. <b><i>Results:</i></b> A total of 118 patients were randomised. The median (IQR) baseline PASI was 8.8 (7.5–11.6). At 24 weeks, the mean difference from baseline in PASI (–1.0 [95% CI –2.0 to 0.0]) was significantly larger in the sitagliptin/NB-UVB arm than in the NB-UVB-alone arm (<i>p</i> = 0.044). There were significant differences in the change in Hospital Anxiety and Depression Scale (–2.5 [95% CI –4.0 to –1.0]; <i>p</i> = 0.002) and EuroQol 5-item questionnaire (0.1 [95% CI 0.0–0.1]; <i>p</i> = 0.036) values from baseline to 24 weeks between the sitagliptin/NB-UVB and the NB-UVB-alone arm. There were no treatment-related serious adverse events. <b><i>Conclusion:</i></b> Sitagliptin therapy combined with NB-UVB phototherapy significantly improved psoriasis severity, albeit modestly, compared to NB-UVB phototherapy alone in patients with moderate psoriasis without T2DM.