Low molar mass (3-17 kg/mol) amino-telechelic polyethylene (ATPE) was used to reactively compatibilize poly(ethylene terephthalate) (PET) and high-density polyethylene (HDPE) via ester aminolysis of PET. A tert-butyloxycarbonyl (Boc)-protected polyethylene precursor was thermolytically deprotected during the melt-blending process to render the reactive amine termini. Spectroscopic analysis of a model reaction confirmed the presence of amide functionality in the resultant material. Through blending studies, we found that low loadings of ATPE (0.5 wt %) significantly reduced the volume of the dispersed HDPE phase particles by a factor of 8 when compared to a binary PET/HDPE blend as assessed by scanning electron microscopy (SEM). Mechanical analysis of the ATPE-compatibilized blends showed a 12-fold increase in the elongation at break over the unmodified PET/HDPE blend. Ultimately, the results here offer a new approach to reactively compatibilize and toughen PET/HDPE blends and open the door for other uses of amino-telechelic polyethylene.