Introduction and Aims: Guided by cognitive theory of loneliness, this study sought to explore the experience of loneliness among people accessing treatment for substance use disorders. Specifically, contributors to, consequences and alleviators of loneliness were explored. Design and Methods: Individual semi-structured interviews were conducted with 20 participants. Interviews were conducted onsite at two residential treatment facilities in New South Wales, Australia. Interviews were audio recorded and transcribed and an iterative categorisation approach was used to guide data analysis and reporting. Results: Four key themes emerged as contributors to and consequences of loneliness: cognitions (mistrust, perceived support from others, low self-worth and fear of negative evaluation), quality and authenticity of relationships, unhelpful interpersonal behaviours and the role of substance use. Participants indicated that overcoming the cognitive and behavioural perpetuators helped to alleviate loneliness and also described the utility of support groups, pursuit of authentic relationships and activities that provide a sense of purpose as helpful. Discussion and Conclusions: Cognitions related to mistrust, lack of perceived support, low self-worth, fear of negative evaluation and identification and pursuit of meaningful relationships supportive of recovery should be key treatment targets for this population.