Platform as a Service (PaaS) solutions are changing the way that software is produced, distributed, consumed, and priced. Unlike Software as a Service or Infrastructure as a Service, PaaS allows for value co-creation by offering complementary components and applications that are developed in emerging ecosystems of third-party developers. Despite increasing interest among practitioners and researchers, there has been little work in understanding how PaaS business models should be designed to establish a flourishing ecosystem. We aim at developing a design theory that facilitates the design of PaaS business models, taking into account the specifics of multisided business models. Four meta-requirements describe the purpose and scope of our design theory, and six design principles guide PaaS providers in designing effective business models. By focusing on designing business models, our research goes beyond previous approaches for studying PaaS.