Methoxy PEGylated nanoparticles (mPEG-NPs) are increasingly used for cancer imaging and therapy. Here we describe a general and simple approach to confer tumor tropism to any mPEG-NP. We demonstrate this approach with humanized bispecific antibodies (BsAbs) that can bind to both mPEG molecules on mPEG-NPs and to EGFR or HER2 molecules overexpressed on the surface of cancer cells. Simple mixing of BsAbs with mPEG-NPs can mediate preferential binding of diverse mPEG-NPs to cancer cells that overexpress EGFR or HER2 under physiological conditions and significantly increase cancer cell killing by liposomal doxorubicin to EGFR(+) and HER2(+) cancer cells. BsAbs modification also enhanced accumulation of fluorescence-labeled NPs and significantly increased the anticancer activity of drug-loaded NPs to antigen-positive human tumors in a mouse model. Anti-mPEG BsAbs offer a simple one-step method to confer tumor specificity to mPEG-NPs for enhanced tumor accumulation and improved therapeutic efficacy.