Although a range of relationship enhancement interventions have shown benefits, programs involving trained facilitators are difficult to scale and self-directed programs tend to suffer from low rates of adherence (i.e., nonuse/disuse attrition). The present study evaluated Agapé, a relationship wellness smartphone app optimized for broad dissemination through 4 years of persuasive system design driven by user feedback. Agapé builds moments of connection by providing daily prompts to couples and then showing them one another's answers when both partners have responded. To evaluate Agapé, a single-arm, pilot study followed 405 couples (n = 810 partners; 68% non-Latinx White; 50% female; M age = 29; 50% cohabiting; 31% married; 33% dissatisfied/distressed) through their first month of using the app. Agapé had high engagement (99%, completing M = 27 daily prompts dyadically) and 88% of couples provided follow-up data. The results among completers supported the acceptability, feasibility, and utility of Agapé, suggesting that partners found the app easy (74%) and enjoyable (93%) to use, particularly if they perceived their partners to be responsive. Most partners saw improvement in their relationships (80%) and in their individual well-being (70%). Repeated-measures analyses of variance (ANOVAs) revealed significant within-person improvement on relationship functioning (decreases in relationship negative qualities and increases in relationship satisfaction, relationship positive qualities, and dedication) and on individual functioning (increases in vitality and quality of life with decreases in psychological distress and depressive symptoms) among completers (Cohen's ds ranging from |.14 to .42|). Dosage effects demonstrated that gains in relationship quality were stronger for couples completing more daily prompts.