Intrastrand thymine dimerization is the dominant process leading to DNA photodamage after ultraviolet irradiation. However, the mechanism behind this photoreaction is not well understood. In this study, the mechanism for photoincluced thymine dimerization has been investigated in the gas phase using high-level ab initio calculations. CASSCF and CASPT2 calculations have been performed to study the potential energy surfaces of two stacked thymines. The excited-state reaction pathway leading to the formation of the mutagenic thymine dimer has been identified and characterized. The central mechanistic feature is a conical intersection between the ground and first excited states along the concerted [2 + 2] cycloaddition pathway. This nonadiabatic reaction path accounts for the experimental observations on the ultrafast thymine dimerization.