Abstract Brachytherapy is a common clinical technique involving implantation of sealed radioactive “seeds” within a tumor to selectively irradiate the tumor mass while minimizing systemic toxicity. To mitigate the disadvantages associated with complex surgical implantation and subsequent device removal procedures, we have developed an alternative approach using a genetically encoded peptide polymer solution composed of a thermally responsive elastin-like polypeptide (ELP) radiolabeled with 131I that self-assembles into radionuclide seeds upon intratumoral injection. The formation of these nontoxic and biodegradable polymer seeds led to prolonged intratumoral retention (∼85% ID/tumor 7 days postinjection) of the radionuclide, elicited a tumor growth delay in 100% of the tumors in two human xenografts (FaDu and PC-3), and cured more than 67% of tumor-bearing animals after a single administration of labeled ELP. These results suggest that in situ self-assembly of biodegradable and injectable radionuclide-containing polypeptide seeds could be a promising therapeutic alternative to conventional brachytherapy. Cancer Res; 72(22); 5956–65. ©2012 AACR.