Despite a growing consensus that 3D printing technology has emerged as a revolutionary driver of innovation in society and has grown exponentially as an innovation ecosystem, our understanding of how the 3D printing ecosystem emerged and has grown is still limited. Based on the case study and the grounded theory approach, we found that four relevant platforms—open-source hardware platforms, online service platforms, free and low-cost 3D design software platforms, and crowdsourcing platforms—nurtured the innovation ecosystem. We also found that complementary relationships among the four platforms accelerated the growth of the ecosystem from the perspective of an increase in the user base, 3D printer sales, 3D design contents, and the emergence of entrepreneurial firms that are developing the platforms. Consequently, we newly suggest “complementary multiplatforms,” a platform strategy that develops multiple platforms with complementary relationships for nurturing an innovation ecosystem. Our findings provide important theoretical grounds for the multiplatform strategy with complementary relationships aimed at facilitating the growth of an innovation ecosystem.