Background <p>Anxiety and depressive disorders are highly prevalent, common mental disorders globally, but access to efficacious, high-reach treatment remains scarce. Although digital cognitive-behavioral therapies (CBTs) demonstrate efficacy and scalability, they remain limited in enhancing positive affect (PA) and reward processing, which are core deficits underlying anhedonia, a key transdiagnostic symptom. This randomized controlled trial (RCT) evaluates a locally tailored, guidance-on-demand, low-intensity Digital Positive Affect Intervention (PAI) developed to improve positive valence systems while alleviating symptoms in adults and university students in Singapore.</p> Methods <p>This single-blind, two-arm, pragmatic RCT will recruit 1,200 community-dwelling adults with mild-to-moderate symptoms of anxiety or depression. Participants will be randomly assigned in a 1:1 allocation to either the Digital PAI or the Self-Monitoring Placebo as the placebo control across six weeks. Digital PAI comprises six weekly 30-minute self-guided online sessions coupled with thrice-daily ecological momentary interventions (EMIs) focusing on PA regulation. The Self-Monitoring Placebo offers non-therapeutic mood-tracking instructions three times daily. The same ecological momentary assessments (EMAs) are administered in both groups after each EMI prompt. Clinical outcome assessments are administered at baseline, mid-treatment (Week 3), post-treatment (Week 6), and 3-month, 6-month, and 12-month follow-ups. Primary outcomes are changes in anxiety and depression severity (Generalized Anxiety Disorder-7 [GAD-7] and Patient Health Questionnaire-9 [PHQ-9]). Secondary outcomes comprise anhedonia, emotion regulation, reward processing, and sleep quality. Randomly selected subsamples will provide wearable data. Analyses will harness multilevel modeling, generalized estimating equations, causal mediation, structural equation modeling, and precision medicine methods to evaluate treatment efficacy, change mechanisms, and moderators.</p> Discussion <p>This RCT examines the proximal (immediate) and distal (long-term) efficacy of a locally tailored PAI, aligned with cultural values and context, that combine PA improvement strategies with scalable digital delivery to integrate skills into daily routines and settings. It fills essential knowledge gaps in digital mental health research by addressing the positive valence system and incorporating prospective EMA and wearable assessments in real-time. If successful, the Digital PAI may inform stepped-care and stratified care models as well as AI-triaging approaches. These efforts may contribute to more extensive implementation of data-driven, patient-centered care in culturally diverse, resource-limited contexts.</p> Trial registration: <p>ClinicalTrials.gov ID (NCT06978257) </p> Date of trial registration: <p>April 15, 2025</p> URL of trial registration: <p><a href="https://clinicaltrials.gov/study/NCT06978257">https://clinicaltrials.gov/study/NCT06978257</a>.</p>