Using solid-state shear pulverization (SSSP) to process poly(ethylene terephthalate) (PET) without addition of chemical agents, we demonstrate that linear PET can be transformed into lightly branched PET, with resulting improvements in physical and mechanical properties. Rheological characterization demonstrates an increase in the melt viscosity of the pulverized PET while intrinsic viscosity characterization yields data consistent with no increase in linear chain length. These results indicate that branching occurs in situ during SSSP via mechanochemistry involving the production of polymeric radicals that result from low levels of chain scission accompanying SSSP. A hypothetical mechanism for this mechanochemical transformation is discussed. The lightly branched PET resulting from SSSP yields a dramatic increase in the crystallization rate of the PET, improving its processability. The ability to increase the melt viscosity of PET by SSSP may contribute to sustainable engineering of PET; a long-standing issue with recycling PET for high-value applications is the fact that melt- processing of PET results in reduction of molecular weight and thereby melt viscosity, making the recycled material often unusable for the original application for which it was made.