Purpose In mobile platforms, an increasing number of third-party developers (developers) create new ideas and enhance their expertise through knowledge sharing on the developers' community. Notwithstanding the importance of the sharing and its uniqueness on the mobile platform contexts, the motivational factors of sharing their knowledge on the community have been underinvestigated. Therefore, this paper aims to provide a comprehensive framework to enhance the knowledge sharing in the mobile platform context. Design/methodology/approach Based on the theory of reasoned action, this paper incorporated intrinsic and internalized extrinsic motivations with two unique features of platform environments: platform open policies and coopetitive relationships. To test, an online survey was distributed to four developer communities in Korea, two Android and two iOS communities. Findings The results show that a platform pursuing higher openness causes the developers to find higher social pressure for information sharing. The coopetitive relationship with other developers in the same platform takes dissimilar roles; reciprocity significantly increases subjective norm, while rivalry does attitude. Self-efficacy and self-development stimulate knowledge sharing. Furthermore, multilevel analysis to capture the difference between two leading mobile platforms indicates no path difference but, interestingly, shows significant mean difference between the two platforms regarding perceived openness and rivalry. Originality/value With this paper, the authors fulfill the need to understand the knowledge-sharing intention of developers in the context of mobile platforms where developers can be potential competitors or cooperators and where two platforms offer different policies and developmental environments.