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Objective Occupational heat exposure (OHE) has been linked with a variety of adverse health effects, however knowledge regarding potential associations with cancer risk is limited. In this study, associations of OHE and breast cancer risk were evaluated in a Spanish population-based case-control study. Methods We enrolled 1,380 breast cancer cases and 1,428 frequency-matched population controls from 10 regions of Spain. A Spanish job exposure matrix, MatEmEsp, was used to assign estimates of OHE as the proportion of workers exposed (P ≥ 25% for at least one year) and proportion of work time with heat stress (according to the wet bulb globe temperature ISO 7243) for each occupation reported over the lifetime. We used three main exposure indices: ever vs. never exposed, lifetime cumulative exposure (heat stress years) and duration of exposure (years). We estimated odds ratios (ORs) and 95% confidence intervals (CIs) applying a lag-period of 5 years, adjusting for potential confounders. Results Approximately 27% of cases and 22% of controls ever had OHE. Highest exposed occupations included industrial workers, miners and laundry workers. For all analyses the reference category was participants who never had OHE. Ever OHE was associated with a higher risk of breast cancer (OR 1.23; 95% CI 1.02, 1.48). Participants in the medium and high tertiles of lifetime cumulative exposure had a higher risk of breast cancer (ORs of 1.22; 95% CI 0.92, 1.63 and 1.41; 95% CI 1.06, 1.87 respectively, p trend = 0.01) as did those with OHE for longer than 10 years (OR 1.36; 95% CI 1.02, 1.81). A stronger association was found for pre-menopause and for hormone receptor positive disease. Conclusion In this large population-based study we found an increased risk of breast cancer associated with OHE. Further research including at other cancer sites is also warranted.