<p>Subjective wellbeing (SWB) is a key social indicator and a potential determinant of health, yet empirical evidence linking neighbourhood social fragmentation to higher SWB remains limited. This study examined the association between neighbourhood social fragmentation and high SWB among 2,056 adults in the Australian Capital Territory using data from the ‘Living well in the ACT Region’ survey. High SWB was assessed using the Personal Wellbeing Index (PWI) and Global Life Satisfaction (GLS). Neighbourhood social fragmentation was measured using the Australian Neighbourhood Social Fragmentation Index (ANSFI). Multilevel logistic regression models were used to examine these associations while adjusting for individual sociodemographic characteristics and area socioeconomic status (SES), measured by the Index of Relative Socioeconomic Advantage and Disadvantage (IRSAD). Higher levels of social fragmentation were associated with lower odds of high SWB in unadjusted models for both PWI and GLS. After adjustments, the association remained significant for high PWI (OR = 0.941, 95% CI: 0.885–0.999) but not for high GLS (OR = 0.955, 95% CI: 0.899–1.016). No statistically significant interaction was observed between ANSFI and IRSAD. Neighbourhood-level variation in high SWB was small: in the null models, neighbourhood differences explained 1.6% of the variance in high PWI and 1.1% in high GLS, and this variation decreased substantially after adjustment (proportional change in variance: 96% for PWI and 52% for GLS). These findings suggest modest associations between neighbourhood social fragmentation and high SWB, with individual-level characteristics explaining much of the neighbourhood-level variation.</p>