Background: Occupational stress is one of the factors affecting social life and is known as a serious threat to the workforce health in the world. Firefighters are subject to frequent occupational stress due to their working conditions and tasks. Objectives: The current study aimed at investigating the effect of occupational stress dimensions on the work ability index of firefighters in a petrochemical company in Iran. Methods: A total of 130 shift-working and day-working firefighters were selected from a petrochemical company using the census method. Occupational stress was evaluated using the health and safety executive (HSE) indicator tool, and the work ability index (WAI) was used to assess the work ability among firefighters. Pearson chi-square test was used to examine the correlation between the various dimensions of occupational stress and the WAI. Results: The mean of occupational stress in the shift-working group was significantly higher than that of the day-working group (P = 0.029). The results of the Pearson correlation test showed a significant relationship between the mean score of occupational stress and that of the WAI (P = 0.01). Among the stressors, the perceived role and managerial support (R = +0.51), and colleagues support (R = +0.50) had the highest correlation with the work ability of firefighters. Conclusions: The obtained results showed that occupational stress can affect the work ability of firefighters. Hence, to improve working conditions, identification of various dimensions of occupational stress, especially among shift workers, and implementation of constructive and managerial interventions should be prioritized.