<p>Mobile app stores are intensely competitive platforms, where the performance of a mobile app is heavily influenced by competitor actions. However, existing literature has paid minimal attention to such competitive effects, focusing instead on absolute measures of mobile app performance such as downloads and ratings. To address this gap, this study analyzes market share—a relative measure of mobile app performance based on a mobile app’s share of downloads relative to its direct competitors. The study identified competing apps by computing cosine similarity on app summaries, a novel method that emphasizes the functional overlap of mobile apps instead of category membership. Drawing on the marketing and competition literatures, the study then developed a comprehensive model to explain market share development in the mobile market context. Estimation results from a beta regression model on a large sample of Play Store apps (N = 23,755) indicate that mobile app quality, portfolio size, mobile app price, and early market entry are positively associated with market share, while portfolio diversification and competitive intensity are negatively associated with market share. The study contributes to the literature by introducing a novel method to identify competing apps, shifting the focus from absolute to relative measures of mobile app performance, and developing a comprehensive model of market share development in the mobile app market context.</p>