Objective: The aim of this study was to examine the prevalence of heat-related illnesses (HRI) and their associated factors among rice farmers in Central Vietnam. Methods: A cross-sectional study was conducted using a sample of 379 farmers from August to September 2021 in Ha Tinh province on the north-central coast of Vietnam. A structured questionnaire was used to collect demographic information, farming activities, heat exposure, and health issues. Multivariable logistic regression was used to identify the factors associated with HRI. Results: At least one HRI symptom was experienced by 83.4% of farmers, and two or more HRI symptoms were experienced by 55.1% of farmers during the last harvesting season (the summer-autumn rice season, 2021). Factors significantly associated with HRI were age, having chronic diseases, type of farming tasks, the number of hours working in the heat, and water intake. Farmers working outdoors for more than 4 hours in the heat were 2.3 times [95%CI: 1.14–4.69] more likely to experience HRI than those working fewer than 2 hours. A higher risk of HRI was also associated with heavy farming tasks (OR = 2.35, [1.34–4.14]), having hypertension (OR = 3.75, [1.90–7.81]), cardiovascular diseases (OR = 2.53, [1.03–6.93]), and other chronic conditions (OR = 2.51, [1.51–4.25]). In contrast, a lower risk of HRI was found in the older group (OR = 0.44, [0.22–0.87]) and increased water intake (OR = 0.46, [0.27–0.77]). Conclusions: Hot weather is a significant occupational health risk for farmers in Vietnam. Changing the working schedule, working in shorter intervals, and remaining well-hydrated should be considered in HRI prevention strategies.