The objective of this study was to employ the theory of planned behavior in examining the inclusion of Islamic religiosity in predicting Islamic ethical work behavior. Islamic religiosity was included as Islam plays a dominant role in Brunei’s society. Participants consisted of 370 Malay Muslim teachers. Structural equation modeling was used to test three proposed models. While Model 1 was based on the theory of planned behavior, it does not take into consideration the distinctive Islamic context of the Bruneian society. Hence, Model 2 was proposed to include Islamic religiosity. To better reflect the population, a model with Islamic religiosity was further augmented leading to Model 3. Results showed support for only two models: Model 1 and Model 3. Model 3 was revised by removing non-significant paths. Model comparison indicated that the revised Model 3 (AIC = 100.82; BIC = 103.55) is a better model than Model 1 (AIC = 141.88; BIC = 143.98). This suggests that the predictive power of the theory of planned behavior can be improved by including dominant cultural factors, such as religiosity, that affect a person’s belief. In conclusion, it can be said that religion is indispensable in exploring and understanding how the people in Brunei think and behave in their workplace and society.