A biocompatible thixotropic hydrogel via photoreduction and self-assembly of silver clusters is described. This reaction avoids use of hydrogelators, crosslinkers or any exogenous toxic reducing agents, making it a facile and green method. Moreover, the formed 'smart' thixotropic hydrogel is pH-responsive and can encapsulate the hydrophobic anticancer drug doxorubicin hydrochloride (DOX). The thixotropic property allows it to be sol state, which can be injected directly into the site of breast tumors in mice; after removal of the shear stress, the sol reverts back to hydrogel, where it remains for delivery of DOX into cancerous cells. The DOX-loaded hydrogel presents remarkable inhibition efficacy towards tumor growth, in addition to reducing the organ toxicity.