Therapeutic constructs with imaging modalities hold great promise for improving the treatment efficacy for cancer and many other diseases. We report here the design and synthesis of a self-assembling prodrug (SAPD) by the direct linkage of camptothecin (CPT), an anticancer drug, to a metal-chelating agent, DOTA. We found that under physiological conditions the DOTA-conjugated CPT prodrug can self-assemble into tubular supramolecular polymers (SPs) with a length of several micrometers. Our studies also suggest that the resultant assemblies were stable in biological environments and exhibited a fast drug release rate in the presence of intracellular glutathione. Furthermore, the SAPD exhibited remarkable in vitro efficacy against various cancer cell lines and effectively inhibited the growth of tumor spheroids. We believe that the design and optimization of self-assembling theranostic conjugates could provide a robust yet simple platform for the development of new imaging-guided drug delivery systems.