Most measures designed to assess eHealth therapeutic alliance (ETA) derive from conventional factors of therapeutic alliance. This study examined whether integrating subscales developed directly to capture therapeutic alliance facets that are unique to the digital space contribute above conventional factors to understanding ETA. The eHealth Therapeutic Alliance Inventory (ETAI) was developed based on a review of face-to-face TA measures combined with new items related to ETA uniquely. Following development, a panel of psychologists who are also eHealth experts, evaluated ETAI’s content validity prior to testing. A sample consisted of 273 adults in the USA, participating in 6-month mobile alcohol reduction intervention who completed ETAI at the end of the intervention. Factor structure was examined using exploratory factor analysis; internal reliability using Cronbach’s α; regressions were calculated assessing items’ contribution to explaining two clinical-related-outcomes: participant experience of positive change, and commitment to change towards the future. Two-factor solution was found with conventional items loading on the first factor and items representing application-induced accountability, on the second, resulting in a 10-item scale with adequate internal consistency for both factors (α = 0.93; α = 0.83, respectively). Both factors had a unique contribution to explaining “experiencing positive change” (factor 1: β = 0.27, p < 0.001; factor 2: β = 0.30, p < 0.001); only factor 2 had a unique contribution to explaining “commitment to change” (factor 2: β = 0.39, p < 0.001). Unique scales capturing ETA may contribute to our understanding of user engagement with digital tools and to explaining clinical-related outcomes.