Theoretically informed measures of eHealth literacy that consider the social affordances of eHealth are limited. This study describes the psychometric testing of a multi-dimensional instrument to measure functional, communicative, critical, and translational eHealth literacies, as informed by the Transactional Model of eHealth Literacy (TMeHL). A 3-phase rating scale construction process was conducted to engage eHealth experts and end-users. In Phase 1, Experts (N = 5) and end-users (N = 25) identified operational behaviors to measure each eHealth literacy dimension. End-users (N = 10) participated in think-aloud interviews to provide feedback on items reviewed and approved by experts. A field test was conducted with a random sample of patients recruited from a university-based research registry (N = 283). Factor analyses and Rasch procedures examined the internal structure of the scores produced by each scale. Pearson's r correlations provided evidence for external validity of scores. The instrument measures four reliable (ω = .92-.96) and correlated (r= .44-.64) factors: functional (4 items), communicative (5 items), critical (5 items), and translational (4 items). Researchers and providers can use this new instrument as a theory-driven instrument to measure four eHealth literacies that are fundamental to the social affordances of the eHealth experience.