Geographic clustering of innovative industries is associated with the entry and success of spinoff firms. We develop a model to explain the multiple empirical patterns regarding cluster growth and spinoff formation and performance, without relying on agglomeration externalities. Clustering naturally follows from spinoffs locating near their parents. In our model, firms grow and spinoffs form through the discovery of new submarkets based on innovation. Rapid and successful innovation creates more opportunities for spinoff entry and drives a region's growth. Our model provides baseline estimates of levels of agglomeration that can be attributed to this process of innovation and spinoff formation.