BACKGROUND: Exposure to excessive heat, which will continue to increase with climate change, is associated with increased morbidity due to a range of noncommunicable diseases (NCDs). Whether this is true for diabetes is unknown. OBJECTIVES: We aimed to quantify the relationship between heat exposure and risk of hospitalization due to diabetes in Brazil. METHODS: Data on hospitalizations and weather conditions were collected from 1,814 cities during the hot seasons from 2000 to 2015. A time-stratified case-crossover design was used to quantify the association between hospitalization for diabetes and heat exposure. Region-specific odds ratios (ORs) were used to calculate the attributable fractions (AFs). RESULTS: A total of 553,351 hospitalizations associated with diabetes were recorded during 2000–2015. Every 5°C increase in daily mean temperature was associated with 6% [OR = 1:06; 95% confidence interval (CI): 1.04, 1.07] increase in hospitalization due to diabetes with lag 0–3 d. The association was greatest (OR = 1:18; 95% CI: 1.13, 1.23) in those ≥80 y of age, but did not vary by sex, and was generally consistent by region and type of diabetes. Assuming a causal association, we estimated that 7.3% (95% CI: 3.5, 10.9) of all hospitalizations due to diabetes in the hot season could be attributed to heat exposure during the study period. DISCUSSION: Short-term heat exposure may increase the burden of diabetes-related hospitalization, especially among the very elderly. As global temperatures continue to rise, this burden is likely to increase. https://doi.org/10.1289/EHP5688.