The effectiveness of mutual help groups (MHGs) in promoting addiction recovery has been widely acknowledged. However, the psychological mechanisms underlying the impact of MHGs remain somewhat uncertain. Drawing on a social identity perspective, this study investigated a sequential mediation model in which social support is posited as a driving factor that enhances abstinence maintenance through group identification, recovery identity, and self-efficacy. A sample of 820 smokers, participating in a 6-month collective smoking cessation programme which included access to an online help group, completed measures of social support, group identification, smoker/ex-smoker identity, and self-efficacy at the programme's outset. Smoking abstinence was assessed 6 and 9 months later. The findings supported the proposed model, indicating that social support was positively associated with MHG identification, which, in turn, was related to a stronger recovery identity. Subsequently, recovery identity was associated with increased self-efficacy, and indirectly, with smoking abstinence at both measurement times. Additional analyses testing alternative mediation models further supported the validity of the proposed model. These findings suggest that social identity factors play significant roles in accounting for the effectiveness of MHGs for addiction recovery.