Ongoing urbanisation and ageing populations pose multifaceted challenges, particularly in high-density cities such as Hong Kong. As the concepts of “Ageing in Place” and “Active Ageing” become more prevalent globally, the role of public spaces in providing social opportunities for older people is more widely discussed. This study has examined how public space features influence the activities and experiences of older adults in high-density urban communities. The research focused on the role of the built environment in active ageing, the levels and needs of social participation among older adults, and the impact of public space features on the social participation of older adults. Through precedent studies, the study identified age-friendly practices around the world to create an integrated framework for the evaluation of the needs for public spaces in the context of Active Ageing. Based on this framework, the project examined the Po Lam and Kin Ming Public Housing Estates in Tseung Kwan O, Hong Kong, through field observations and interviews with older residents. The research has identified key factors influencing the social participation of older adults in high-density urban areas, including pleasant and clean environments, community facilities, places supporting social interaction, access to nature, and accessibility. The paper concludes with urban design and planning recommendations for promoting the social participation of older adults in Hong Kong. Ultimately, this work contributes to the creation of more age-friendly environments that support social and psychological well-being.