**Background.** With a rapidly ageing society, healthy ageing has become a key challenge. Engagement in physical activity, and particularly walking, is a key strategy that contributes to healthy ageing amongst older adults. The purpose of the present study was to evaluate the efficacy of a group walking program for older adults that incorporates the 5R Shared Leadership Program (5R^S). By implementing a structure of shared leadership and strengthening peer leaders’ identity leadership, 5R^S aims to cultivate a shared social identity amongst participants, which has in other contexts been associated with greater performance and well-being. **Methods.** A cluster randomised controlled trial was conducted to test the efficacy of the 5R^S group walking program on group identification, group cohesion, walking activity, and well-being, compared to a regular group walking program for older adults. Nineteen older adult walking groups (i.e., the clusters; N = 503; *M_age* = 69.23 years, *SD* = 6.68) all participated in a 12-week structured group walking program. Nine walking groups (*n* = 304) were randomly assigned to the intervention in which participants received the 5R^S program in addition to regular group walking. **Results.** 5R^S was successful in strengthening the identity leadership qualities of the appointed peer leaders. Multilevel regressions showed that 5RS succeeded in increasing group cohesion and walking activity to a greater extent than a regular group walking program, while participants’ group identification and well-being increased to a similar extent in both conditions. Furthermore, structural equation modelling revealed that group identification mediated the impact of peer leaders’ identity leadership on group cohesion and well-being (but not walking activity). **Conclusion.** By harnessing the capacity of the group and its peer leaders, the 5R^S program offers a promising intervention to engage older adults in physical activity. **Trial registration.** The study was retrospectively registered as clinical trial on 9 September 2021 (NCT05038423).