Singlet fission, the generation of two triplet excited states from the absorption of a single photon, may potentially increase solar energy conversion efficiency. A major roadblock in realizing this potential is the limited number of molecules available with high singlet fission yields and sufficient chemical stability. Here, we demonstrate a strategy for developing singlet fission materials in which we start with a stable molecular platform and use strain to tune the singlet and triplet energies. Using perylene diimide as a model system, we tune the singlet fission energetics from endoergic to exoergic or iso-energetic by straining the molecular backbone. The result is an increase in the singlet fission rate by 2 orders of magnitude. This demonstration opens a door to greatly expanding the molecular toolbox for singlet fission.