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Objectives This analysis aimed to confirm the reliability, validity, and responsiveness of the Psoriasis Symptom Diary (PSD) using data from two Phase III studies in patients with moderate to severe chronic plaque psoriasis. Methods Data from two randomized, double-blind, double-dummy, placebo-controlled, multicenter Phase III studies (n = 820) assessing the efficacy and safety of secukinumab were used. The PSD (24-h recall; 0–10 numeric rating scale) was electronically administered each evening. Test–retest reliability was determined using intraclass correlations. Construct validity hypotheses were evaluated via correlations with the Psoriasis Area and Severity Index (PASI), Investigator's Global Assessment (IGA), Dermatology Life Quality Index (DLQI), EuroQoL 5-Dimension Health Status Questionnaire, and Patient Global Impression of Change (PGIC). Discriminating ability and responsiveness were evaluated by estimating mean differences and effect sizes between known groups (using the PASI and IGA). Phase II-derived, anchor-based PGIC thresholds and cumulative distribution function (CDF) plots described meaningful change. Results Items on the PSD yielded high intraclass coefficients (>0.90). Correlations were in the anticipated direction and by week 12 were moderate to strong (0.41–0.73) in magnitude, demonstrating construct validity. Average PSD item scores differed predictably and significantly between known groups. Responsiveness effect size estimates were moderate to large (0.6–1.5), and CDF plots showed the percentage of responders to be consistently higher in treatment than in placebo arms across the range of change in PSD scores. Conclusions The PSD is reliable, valid, and responsive, and represents a valid tool to enhance treatment decisions in patients with moderate to severe plaque psoriasis.