There are well-established relationships between socioeconomic status (SES) and all aspects of health. Brief interventions offer a cost-effective method to target health behaviours, helping to reduce these health inequalities. Furthermore, the third and social economy (TSE) sector, which encompasses all not-for-profit groups and organisations that operate outside of the family, household, and government, offers access to those of lower SES and a motivated workforce with established relationships with service users. The aim of this systematic review was to investigate the effectiveness of brief interventions targeting health behaviours and their social determinants, when delivered within TSE settings or by TSE service providers (PROSPERO registration number: CRD42022301969). Eight databases were searched for brief health behaviour change interventions lasting under 30 minutes per session, delivered by volunteers within the TSE sector or delivered within the TSE sector provided by or within a TSE, from all possible publication dates to February 2022. Behaviours relating to smoking, diet, alcohol, physical activity, housing, or finance were included. Narrative synthesis and Cochrane risk of bias tools were applied. Eight eligible studies were identified, most measuring smoking behaviour and with a considerable risk of bias. Only one study was set both within a TSE setting and delivered by TSE providers. The most common behaviour change techniques applied were the provision of information on both the consequences of the behaviour and further support. Brief interventions showed a minimal, if any, reduction in smoking behaviour and cholesterol levels, with more intensive interventions resulting in a far greater improvement in smoking, diet, and physical activity behaviours than brief intervention. This study highlighted a lack of research on brief interventions within the TSE sector, particularly for alcohol consumption. More qualitative research is needed to explore the feasibility and acceptability of brief interventions within the TSE settings. Limitations are discussed, including the high risk of bias of included studies and the exclusion of mental health.