Loneliness is especially frequent among older people in Southern Europe. Furthermore, promoting social capital to tackle loneliness and its health effects is an understudied intervention strategy. Therefore, a complex intervention was piloted in Spain in a pre-post study with a 2-year follow-up. Its aims were to explore the feasibility of the intervention and its short- and long-term effects. It was conducted in one mixed rural-urban and two urban areas of diverse socioeconomic levels from 2011 to 2012. The intervention framework was based on social capital theory applying a behaviour change model and care co-ordination. The intervention comprised: (i) a co-ordinated action aimed at building a network between primary healthcare centres and community assets in the neighbourhood and (ii) a group-based programme, which promoted social capital among lonely older people, especially social support and participation. Older people active in senior centres volunteered as gatekeepers. The main outcome domain was loneliness. Secondary outcome domains were participation, social support, self-perceived health, quality of life, depressive symptoms and use of health resources. Pre-post changes were assessed with t-test, Wilcoxon signed-rank test and McNemar's test. Differences between the three time points were assessed with a one-way ANOVA with repeated measures. Social workers and nurses were successfully involved as group leaders, 10 volunteers took part and 38 participants were included. After the intervention, loneliness decreased while social participation and support significantly increased. Furthermore, the number of visits to nurses increased. Exactly 65.8% of the participants built social contacts within the group and 47.4% became engaged in new activities. Two years later, social effects were maintained and depressive symptoms had decreased. Exactly 44.7% of the participants continued to be in contact with at least one person from the group and 39.5% continued participating. The intervention contributes a novel and feasible social capital-based approach for alleviating loneliness among older adults while prompting meaningful changes in their lives.