A convenience sample of 234 older adults living alone will be recruited from six participating community centres (39 participants from each centre). Eligible older adults will be randomly assigned to a waitlist control group (n = 117) or an intervention group (n = 117). Both groups will receive usual community support services. The intervention group will additionally receive four weekly face-to-face ACT-based intervention sessions (approximately 90 minutes each) in group format (each group consisting of six to eight older adults).
The primary outcome (loneliness) and secondary outcomes (psychological flexibility, psychological distress, health-related quality of life, and healthcare utilisation) will be assessed at baseline (T0), immediately post-intervention (T1), 3 months post-intervention (T2) and 6 months post-intervention (T3), using validated questionnaires. A purposive subsample of 36 intervention group participants will be invited for focus group interviews. A generalised estimating equation model with intention-to-treat and path analyses will be used to analyse the quantitative data, whereas thematic analysis will be used for the qualitative data.