Sexual wellbeing is an important aspect of population health. Addressing and monitoring it as a distinct issue requires valid measures. Our previous conceptual work identified seven domains of sexual wellbeing: security; respect; self-esteem; resilience; forgiveness; self-determination; and comfort. Here, we describe the development and validation of a measure of sexual wellbeing reflecting these domains. Based on the analysis of 40 semi-structured interviews, we operationalized domains into items, and refined them via cognitive interviews, workshops, and expert review. We tested the items via two web-based surveys (n = 590; n = 814). Using data from the first survey, we carried out exploratory factor analysis to assess and eliminate poor performing items. Using data from the second survey, we carried out confirmatory factor analysis to examine model fit and associations between the item reduced measure and external variables hypothesized to correlate with sexual wellbeing (external validity). A sub-sample (n = 113) repeated the second survey after 2 weeks to evaluate test-retest reliability. Confirmatory factor analysis indicated that a "general specific model" had best fit (RMSEA: 0.064; CFI: 0.975, TLI: 0.962), and functioned equivalently across age group, gender, sexual orientation, and relationship status. The final Natsal-SW measure comprised 13 items (from an initial set of 25). It was associated with external variables in the directions hypothesized (all p < .001), including mental wellbeing (0.454), self-esteem (0.564), body image (0.232), depression (-0.384), anxiety (-0.340), sexual satisfaction (0.680) and sexual distress (-0.615), and demonstrated good test-retest reliability (ICC = 0.78). The measure enables sexual wellbeing to be quantified and understood within and across populations.