The complex set of challenges that middle-aged adults encounter emphasizes a need for mental health interventions that promote resilience and positive outcomes. The present study evaluated whether an online, self-guided social intelligence training (SIT) program (8 h) improved midlife adults' daily well-being and emotion regulation in the context of their own naturalistic everyday environment. A randomized controlled trial was conducted with 230 midlife adults allocated into either a SIT program or an attentional control (AC) condition that focused on healthy lifestyle education. Intent-to-treat analyses examined two bursts of 14-day daily surveys that participants completed pre- and post-treatment. Multilevel models evaluated pre-to post-treatment changes in mean positive and negative affect, as well as daily emotional reactivity to stressors and responsiveness to uplifts. Compared to the AC group, those in the SIT program reported improvements (i.e., decreases) in mean negative affect, positive emotional reactivity to daily stressors (i.e., smaller decreases in positive affect on stressor days), and negative emotional responsiveness to uplifts (i.e., lower negative affect on days without uplifts). Our discussion considers potential mechanisms underlying these improvements, highlights downstream effects on midlife functioning, and elaborates on how online delivery of the SIT program increases its potential for positive outcomes across adulthood.