Objective: This research aimed to examine the effectiveness of an Android mobile game application called Arbi Care as a means to prevent diarrhea and build self-efficacy in hand washing among preschool children. Method: This research used a pre- and post-test control group and time series design approach. Respondents were chosen randomly from a group of four to six years children. The intervention group (n = 60) received Arbi Care intervention for 25 minutes, twice a week, for five weeks while the control group (n = 60) received standard education. Self-efficacy was measured by using questionnaire and observation. Measurement was carried out three times in the sixth, eight, and tenth week post-intervention. The data was analyzed using the GLMRM test. Results: There was a significant increase in the average score of self-efficacy in hand washing for the intervention group versus the control group. Moreover, there were significant differences in the results of average scores in which the intervention group showed much better self-efficacy improvement over the control group during the first, second, and final post-test after the intervention was given (p < 0.001). Conclusions: An Android-based educational game can be an effective medium to improve hand washing self-efficacy among preschool children, thus helping to prevent diarrhea.