Poly(ethylene terephthalate) (PET)/Cloisite 30B (C30B) nanocomposites of different organoclay concentrations were prepared using a water-assisted extrusion process. The reduction of the molecular weight (Mw) of the PET matrix, caused by hydrolysis during water-assisted extrusion, was compensated by subsequent solid-state polymerization (SSP). Viscometry, titration, rheological, and dynamic scanning calorimetry measurements were used to analyze the samples from SSP. The weight-average molecular weight (Mw) of PET increased significantly through SSP. PET nanocomposites exhibited solid-like rheological behavior, and the complex viscosity at high frequencies was scaled with the Mw of PET. The Maron-Pierce model was used to evaluate the Mw of PET in the nanocomposites before and after SSP. It was found that the extent and the rate of the SSP reaction in nanocomposites were lower than those for the neat PETs, due to the barrier effect of clay platelets. Consequently, the SSP rate of PET increased with decreasing particle size for the neat PET and PET nanocomposites. The effect of the Mw of PET on the crystallization temperature, crystallinity, and the half-time, t1/2, of nonisothermal crystallization was also investigated. With increasing Mw of PET, t1/2 increased, whereas Tc and Xc decreased.