Persuasive technology through interactive applications on wearable smartwatches holds great promise in the physical health domain. Its effectiveness for promoting physical activity might be further enhanced by social support strategies (such as competition and cooperation) from the Persuasive System Design (PSD) framework, particularly the combination of both elements in the form of intergroup competition. This study developed a smartwatch fitness application for the HarmonyOS platform with the core feature of intergroup competition, and evaluated its usability and effectiveness in the real-world setting through a two-week-long experiment on 16 smartwatch users. Under a within-subjects design, the field study assessed and compared daily exercise performance, attitudes, as well as user experiences with the application, based on the teammate relationship type (i.e., whether with a friend or a stranger). Results showed that intergroup competition effectively promoted the weekly total exercise time to an average of over 100 min regardless of teammate relationship type, while each type influenced exercise motivations and attitudes differently. The application also received positive user feedback, particularly from those new to smartwatches. This study provided empirical support for the use of smartwatch-based persuasive design to encourage healthier behaviors, and suggested the broader applicability of intergroup competition in digital interventions for increasing physical activity.