What are the targets of natural selection remains a controversial issue in Biology. Here I propose the “Extended Fitness” hypothesis, in which extended phenotypes emerge as a link between individual and group selection. The basic premise of the extended fitness hypothesis is that extended phenotypes can be used by members of the same group since they are adapted to use them. Thus, extended phenotypes can also contribute to the fitness of members of the same species. Group selection emerges as a natural consequence of the shared use of extended phenotypes, which allow the evolutionary forces acting on individuals to become effective at the group level. The extended fitness hypothesis is supported by several observations found in nature. The hypothesis presented here provides a theoretical framework for the design of both modeling and experimental approaches to studying the role of extended phenotypes in group selection.