Metabolic syndrome (MetS) is an important and priority public health problem globally. Long working hours have been proposed as a modifiable risk factor for MetS, despite sparse epidemiological evidence. Thus, the aim of this study was to prospectively evaluate the associations between working hours and incidence of MetS and each of its components.We assessed 6845 participants of a Spanish dynamic prospective cohort of university graduates (the SUN project), initially free of any specific criteria of MetS, and followed-up for a median of 8.3 years. Weekly working hours were collected at baseline and grouped into four categories: >0-24, 25-39, 40-49 and ≥50 h. MetS was defined according to the updated harmonizing criteria. We estimated multivariable adjusted Relative Risks (RR) of MetS and their 95% Confidence Intervals (95% CI), using Poisson regression models.The cumulative incidence of MetS was 6.0%. Working hours were not independently related to MetS (25-39 h/week = RR: 1.42, 95% CI 0.90-2.25; 40-49 h/week = RR: 1.45, 95% CI 0.91-2.30; ≥50 h/week = RR: 1.49, 95% CI 0.91-2.42, P for trend = 0.235) nor to any of its individual definition criteria.Our findings do not suggest that long working hours increase the risk of MetS development or each of its components. Further longitudinal studies in general population should be conducted to confirm these results.