Unpaid overtime-describing a situation where extra hours are worked but not paid for-is a common feature of the labor market that, together with other forms of wage theft, costs workers billions of dollars annually. In this study, we examine the association between unpaid overtime and mental health in the Canadian working population. We also assess the relative strength of that association by comparing it against those of other broadly recognized work stressors. Data were drawn from a survey administered to a heterogeneous sample of workers in Canada (n = 3691). Generalized linear models quantified associations between unpaid overtime, stress, and burnout, distinguishing between moderate (1-5) and excessive (6 or more) hours of unpaid overtime. Unpaid overtime was associated with higher levels of stress and burnout. Relative to those working no unpaid overtime, men working excessive unpaid overtime were 85% more likely to report stress (prevalence ratios [PR]: 1.85, 95% confidence interval [CI]: 1.26-2.72) and 84% more likely to report burnout (PR: 1.84, 95% CI: 1.34-2.54), while women working excessive unpaid overtime were 90% more likely to report stress (PR: 1.90, 95% CI: 1.32-2.75) and 52% more likely to report burnout (PR: 1.52; 95% CI: 1.12-2.06). The association of excessive unpaid overtime with mental health was comparable in magnitude to that of shift work and low job control. Unpaid overtime may present a significant challenge to the mental health of working people, highlighting the potential role of wage theft as a neglected occupational health hazard.