Erection hardness is a fundamental component of erectile function, and is a very specific and easily monitored outcome. The Erection Hardness Score (EHS) is a single-item, patient-reported outcome (PRO) for scoring erection hardness. The aim of this article is to report the psychometric validation of the EHS. The dataset (N = 307) was from a multinational sildenafil trial (efficacy in the treatment of erectile dysfunction [ED]) with a 2-week screening phase, a 6-week double-blind, placebo-controlled treatment phase, and a 6-week open-label extension. Test-retest reliability (intraclass correlation coefficient), quality and distribution of responses, known-groups validity (ability to differentiate between ED severity groups defined by the International Index of Erectile Function [IIEF] questionnaire), convergent validity (Pearson correlation coefficients with domain scores of the IIEF and the Quality of Erection Questionnaire [QEQ]), treatment responsiveness, and clinically important difference. The EHS demonstrated good test-retest reliability, acceptable quality and distribution of responses, known-groups validity against the IIEF (including clear differentiation between normal and impaired erectile function), moderate-to-strong convergent validity against the prespecified domains of the IIEF and QEQ, and high treatment responsiveness. The EHS has desirable measurement properties, including being highly responsive to treatment. This one-item PRO is robust and easy to use for evaluating erection hardness. Psychometric analysis supports the use of the EHS as a simple, reliable, and valid tool for the assessment of erection hardness in clinical trials research.