BACKGROUND: Under the definition of positive mental health, the present study focused on the emerging group of floating elderly to explore the impact and mediating mechanisms of bonding and bridging neighbourhood social capital on their mental health. METHODS: The data were sourced from community surveys in three major cities in Guangdong Province, China, with a total of 659 respondents aged 55 and above. Structural equation modelling was used to verify the hypotheses proposed in this study. RESULTS: The latent variables of bonding and bridging neighbourhood social capital were established. After controlling for the main demographic and social characteristic variables, bonding neighbourhood social capital had a significant positive impact on the happiness of floating elderly (beta=0.484, p<
0.05). Bridging neighbourhood social capital was beneficial for alleviating the tendency towards mental illness (beta=-0.545, p<
0.05). Physical exercise and psychological integration were mediating mechanisms that connected bonding neighbourhood social capital and happiness (RIT=0.313, p<
0.05
RIT=0.674, p<
0.001). Social adaptation was found to have a mediating effect on the relationship between bridging neighbourhood social capital and mental illness (RIT=0.088, p<
0.05). CONCLUSIONS: By incorporating the geographical concept of neighbours into the discussion, the findings support the utility of social capital theory in the context of the floating elderly population and Chinese cities. By classifying neighbourhood social capital, the impact of bonding and bridging social capital on different dimensions of mental health is revealed, further expanding the mechanism of the mental health effect of social capital. Cultivating neighbourhood social capital and transforming individual interaction needs into practical actions should be included in the construction of mental health promotion policies.