Fabrication of self-reinforced polyethylene terephthalate (PET) has been achieved through the in situ generation of PET fibrils via a spun bond process. The reinforcement fibrils created from the PET with higher Tm are made from a unique in situ processing method. As a result, the fibrils are well dispersed and distributed in the lower Tm PET matrix. The high degree of molecular similarity affords perfect interfaces between the matrix and dispersed phase, leading to excellent stress transfer from the matrix to the dispersed fibrils. While the extremely large interfaces from the nanofibrillation process can maximize the advantage of the excellent molecular similarity of the self-reinforced polymeric composites, few studies have been conducted to research nanofibrillar self-reinforced polymeric composite systems. Hence, as a proof of concept, this work provides new insight into an approach for developing a self-reinforced polymeric system with a nanofibrillation process. This process increases the tensile strength of PET composites by up to 15% compared to composites made by a simple blending process and 47% higher than neat PET. Furthermore, extensional viscosity measurements show a strain-hardening behavior in the fibrillated PET composites not observed in the neat PET and showed minimal behavior in un-fibrillated PET composites. The foam process results reveal that the presence of PET fibrils in PET improves the expansion ratio as well as the cell density of the PET composites. Specifically, compared to the PET composite foams without the fibrillation process, fibrillated PET composite foams showed up to 3.7 times higher expansion ratios and one to two orders of magnitude higher cell densities. In thermal conductivity measurements, fibrillated PET composite foams achieved thermal conductivity of as low as 0.032 W/mK.