Walking is an essential mode of travel for older adults, offering significant health benefits. However walking as a travel mode in Vietnam is constrained by poor built environment quality and safety, which act as a psychological barrier to pedestrians in a traffic saturated environment. This study explores an extension of the Theory of Planned Behaviour (TPB) by incorporating built environment quality and safety constructs, along with proximity to destination types and vehicle availability indicators. A survey of active older adults (n = 832, aged 55 to 72) was designed and administered within four inner districts in Ho Chi Minh City (HCMC) to elicit socio-demographic, travel characteristics and psychometric data about past walking behaviour and future intention. A partial-least squares structural equation model (PLS-SEM) was used to conduct path and multi-group analyses (MGA) on main activity segments (retired, working at home and working outside), revealing statistically significant paths with satisfactory variance explained in a conceptual extended Theory of Planned Behaviour (eTPB) framework. The results show that intention to walk can be explained by past behaviour, with mediating effects from other factors, such as built environment and safety, via the standard TPB constructs (attitudes, subjective norms and perceived behavioural control). A clear relationship emerges between the factors examined, but with some exceptions and difference in MGA. This study demonstrates the importance of understanding older adults through examination of their main activity status. We also call for the development of travel behaviour and urban intervention programs to improve walking uptake and safety for older adults.