Background: Collecting reliable and valid symptom information from patients is critical for assessing psoriasis severity in clinical research. Objective: To evaluate measurement properties of a new patient-reported outcome (PRO), the Psoriasis Symptoms and Signs Diary (PSSD). Methods: One hundred six US patients with moderate-to-severe plaque psoriasis completed two versions of the PSSD [a 24-hour recall (PSSD-24h) and 7-day recall (PSSD-7d)] using a 0–10 numerical rating scale. Reliability (test-retest and internal consistency), validity (convergent, divergent and known-groups), responsiveness, and version equivalence were evaluated. Minimally important difference was estimated. Results: Based on exploratory factor analysis and clinical input, symptom, sign, and total severity scores were established. Internal consistency (Cronbach’s alpha ≥ 0.944) and test-retest reliability (intraclass correlation coefficients ≥ 0.824) were acceptable. Correlations with Dermatology Life Quality Index (DLQI) (0.489 to 0.644) indicated convergent validity, while low correlations (Limitations: PSSD responsiveness should be evaluated in patients receiving treatment. Conclusion: The PSSD is reliable and valid in measuring symptoms/signs of patients with moderate-to-severe plaque psoriasis.