Loneliness is a pressing public health issue. Although quintessentially individual, it is shaped by wider environmental, cultural, socio-economic, and political circumstances. Using a systematic review methodology, this paper draws on interdisciplinary research to conceptualise the relationship between the built environment and loneliness. We present a narrative synthesis of 57 relevant studies to characterise the body of evidence and highlight specific built-environment elements. Our findings demonstrate the need for further conceptual and empirical explorations of the multifaceted ways in which built environments can prevent loneliness, supporting calls for investment into this public-health approach.