Inequalities in young people's mental health have been documented according to social class but less is known about determinants that can buffer or mediate the relationship. Social capital has the potential to contribute to alleviating observed health inequalities. However, clarity about how it can be understood and measured in relation to mental health among younger populations remains inconsistent. This scoping review examined published literature to investigate how social capital has been researched for young people's mental health. An established framework was used to guide the methodology. Studies were included: on age (10-19 years); publication year (since 2000); language (English). Only studies using social capital as a central theme were included. No restriction was placed on mental health outcomes. Nine bibliographic databases were interrogated. Articles (1541) were screened, 793 retained for analysis and 73 articles were included. Most studies were conducted in North America and Europe. Twenty per cent provided insights into how social capital should be described in relation to young people. A majority of the studies provided links between varying social capital indicators and a range of mental health outcomes (70%), however such evidence was associational. Only few studies inferred the causal direction between social capital and health (10%) and there were no dedicated studies on measurement. Findings suggest that literature on social capital and young people's mental health has grown but continues to be variously described and measured. It requires better utilization of existing knowledge and new research to improve its application in practice. © The Author(s) 2020. Published by Oxford University Press. All rights reserved.