Background Existing tools for nail psoriasis are complex and may not adequately measure outcomes that are important to patients. Objectives We have developed and validated a new tool, the Nail Assessment in Psoriasis and Psoriatic Arthritis (NAPPA), with three components: a questionnaire assessing quality of life (NAPPA-QoL), a two-part questionnaire assessing patient-relevant treatment benefits (the Patient Benefit Index, NAPPA-PBI) and a psoriasis Clinical Assessment of Severity (NAPPA-CLIN). Methods Development of the questionnaires involved multiple steps: (i) collection of items about nail psoriasis-related impairments and treatment goals; (ii) selection of 48 items by an expert panel, including patients; (iii) translation into eight languages; (iv) feasibility testing and (v) longitudinal validation in six countries. Results Patients found the questionnaires clear (84%) and comprehensible (95%). NAPPA-QoL and NAPPA-PBI scores correlated moderately with clinical outcomes [e.g. Nail Psoriasis Severity Index (NAPSI)] and markedly with other quality-of-life questionnaires (e.g. EQ-5D™). Both questionnaires were sensitive to change. Internal consistency was good (Cronbach α ≥ 0·88 for all scales). The NAPPA-CLIN, a brief version of NAPSI that involves assessment of only four digits rather than all 20, was found to correlate highly with total NAPSI score (r = 0·97, P < 0·001). Conclusions Overall, the three-component NAPPA tool is a valid, reliable and practical instrument to assess patient-relevant nail psoriasis outcomes. What's already known about this topic? Existing tools for assessment of nail psoriasis are complex and may not adequately measure outcomes that are important to patients. What does this study add? We developed and validated a new tool, the Nail Assessment in Psoriasis and Psoriatic Arthritis (NAPPA), which assesses quality of life (NAPPA-QoL), patient-relevant treatment benefits (NAPPA-PBI) and nail psoriasis severity (NAPPA-CLIN). The NAPPA tool is valid, reliable and practical for use in clinical and research settings to assess patient-relevant nail psoriasis outcomes. © 2013 British Association of Dermatologists.