Little is known about the relationship between nail psoriasis and psoriasis severity in children, and there has been no longitudinal assessment of psoriasis severity related to nail psoriasis. The aim of this study was to assess whether nail psoriasis could serve as a predictor for a more severe disease course. De-identified data were obtained from the ChildCAPTURE registry, a daily clinical practice cohort of children with psoriasis, from September 2008 to November 2015. Cross-sectional analyses were performed at baseline. Longitudinal data until 2-year follow-up were analysed by linear mixed models. Nail psoriasis was present in 19.0% of all 343 patients at baseline and cross-sectionally associated with higher Psoriasis Area and Severity Index (PASI) (p = 0.033). Longitudinal analysis demonstrated higher PASI (p<0.001) during 2-year follow-up in patients with nail involvement at baseline. These findings suggest that nail psoriasis is a potential clinical predictor for more severe disease course over time in paediatric psoriasis.