Water-soluble, doxorubicin (DOX) conjugated gold nanoparticles (DOX conjugated Au NPs) exhibiting a significant pH-responsive drug release profile have been prepared and characterized in this study. The Au NPs were stabilized by thiolated methoxy polyethylene glycol (MPEG-SH) and methyl thioglycolate (MTG) at an equal molar ratio. The anticancer drug DOX was conjugated to the MTG segments of the thiol-stabilized Au NPs using hydrazine as the linker. The resulting hydrazone bonds formed between the DOX molecules and the MTG segments of the thiol-stabilized Au NPs are acid cleavable, thereby providing a strong pH-responsive drug release profile. The MPEG segments attached to the Au NPs provide the Au NPs with excellent solubility and stability in an aqueous medium while potentially enhancing the circulation time. The DOX loading level was determined to be 23 wt.%. The DOX release rate from the DOX conjugated Au NPs in an acid medium (i.e., pH 5.3) was dramatically higher than that in physiological conditions (i.e., pH 7.4). The DOX conjugated Au NPs and/or the DOX released from them were found both at the perinuclear regions and the nuclei of 4T1 tumor cells after incubation in a DOX conjugated Au NPs solution for 28 h. These novel DOX conjugated Au NPs have the potential to simultaneously enhance CT imaging contrast and facilitate photothermal cancer therapy while delivering anticancer drugs to their target sites.