An inactive lifestyle is associated with an increased risk of health problems. The combination of mobile step-trackers and persuasive strategies can be considered useful tools for encouraging physical activity. This paper presents the design, development, and evaluation of a culturally tailored persuasive app to motivate physical activity. For this research, we developed a step-tracking app, StepsBooster-S, that is tailored to be culturally appropriate for Saudi adults using the user-centred design approach. A 10-day in-the-wild study was conducted with 30 participants to evaluate the usability and effectiveness of the app using a mixed-methods approach. Results showed that StepsBooster-S is generally effective; however, it led to a highly significant increase in physical activity among the Saudis compared to Canadians. Our results also showed that the Saudi audience engaged more with the app, reported more positive experience from using the app, and enjoyed the collectivists-oriented features such as cooperation more than the Canadian audience. We conclude that persuasive health apps, especially those that are targeted at physical activity, are more effective if they are tailored to be culturally appropriate for the target audience. These findings reinforce the importance of cultural factors for designing technologies that motivate behaviour change.