Research suggests that the financial crisis of 2008 and its aftermath were associated with an increase in mental health problems, but there has been little research into potential protective factors. Ecological social capital is a plausible candidate given evidence of its protective status following natural disasters. Pre‐crisis area‐level estimates of generalized trust and sense of belonging were computed from the 2004 to 2006 waves of the Living in Wales survey (N = 43,473) for 413 neighbourhoods in Wales, using multilevel regression with post‐stratification, a technique for disaggregating survey data into small area estimates. Area estimates and the planned analysis were preregistered using Open Science Framework. Data (N = 180,462) from the Welsh Health Survey (2003–2015) were then used to model whether social capital was protective against depression in general and whether it moderated the increase in depression prevalence, associated with the financial crisis. Depression rates increased post‐crisis (odds ratio [OR] = 1.271), and trust was a protective factor against depression (OR = 0.775). The hypothesized interaction, however, was not significant (OR = 1.033), nor was sense of belonging (OR = 0.934) or its interaction with the crisis (OR = 1.024). Although ecological generalized trust appears to be a protective factor against depression, it did not buffer against the mental health impact of the financial crisis.