Social capital is a widely used concept in the social sciences. Although the quality of social relationships is an important dimension of social capital, most empirical studies primarily focus on its structural dimensions. The aim of this article is to investigate the association between structural social capital, the quality of social relationships, and self-reported depression. The central question is whether there is a difference between structural and qualitative dimensions of social capital in relation to self-reported depression. The data come from the Belarusian National Health Survey, which includes a nationally representative sample of 2107 individuals. Five different forms of social capital were measured, encompassing both structural and qualitative dimensions. The findings show that the most important forms of social capital for mental health among Belarusians are informal relationships with family, friends, and neighbours. Moreover, and of particular relevance to this study, the quality of social relationships are more strongly associated with depression than their structure. To better align with the theoretical framework of social capital, the article concludes that a combination of structural and qualitative indicators is essential when measuring social capital. Including qualitative dimensions may also be important for revealing the potential negative (mental health) outcomes of social capital.