Development of biodegradable nanomaterials for drug delivery and cancer theranostics has attracted great attention in recent years. In this work, polydopamine (PDA), a biocompatible polymer, is developed as a promising carrier for loading of both radionuclides and an anticancer drug to realize nuclear-imaging-guided combined radioisotope therapy (RIT) and chemotherapy of cancer in one system. It is found that PDA nanoparticles after modification with poly(ethylene glycol) (PEG) can successfully load several different radionuclides such as Tc-99m and I-131, as well as an anticancer drug doxorubicin (DOX). While labeling PDA-PEG with Tc-99m (Tc-99m-PDA-PEG) enables in vivo single photon emission computed tomography imaging, nanoparticles co-loaded with I-131 and DOX (I-131-PDA-PEG/DOX) can be utilized for combined RIT and chemotherapy, which offers effective cancer treatment efficacy in a remarkably synergistic manner, without rendering significant toxicity to the treated animals. Therefore, this study presents an interesting class of biocompatible nanocarriers, which allow the combination of RIT and chemotherapy, the two extensively applied cancer therapeutic strategies, promising for future clinic translations in cancer treatment.