Precise local delivery of chemotherapeutic agents employing an injectable depot could be a promising approach to achieve spatiotemporal control over the drug release along with minimizing the challenges associated with the systemic delivery of chemotherapeutic agents. In this regard, we report the development and evaluation of a poly(ethylene glycol) (PEG) hydrogel-based drug delivery platform for the covalent entrapment and sustained release of chemotherapeutic agents. The hydrogels were fabricated by cross-linking of 8-arm PEG glyoxylic aldehyde and 8-arm PEG hydrazine using glyoxylic hydrazone linkages, without employing small-molecule cross-linkers. The hydrogels displayed pH-responsive gelation and swelling pattern along with mechanical robustness, with storage modulus of up to 1650 Pa. Owing to the reversible nature of glyoxylic hydrazone linkages, hydrogels exhibited excellent thixotropic and self-healing characteristics. Doxorubicin (DOX) covalently entrapped into the hydrogel matrix by attaching it to 8-arm PEG hydrazine in substoichiometric ratios, prior to fabrication of hydrogels. A controlled release of up to 81.33% DOX was obtained from 5% hydrogels after 40 days at tumoral pH (6.4 +/- 0.05) and only 42.87% DOX at physiological pH (7.4 +/- 0.05). The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and three-dimensional cell encapsulation studies using NIH-3T3 cell lines demonstrated the biocompatible nature of polymers as well as the hydrogel matrix. The multicellular tumor spheroid growth suppression studies demonstrated a 40.50% reduction in tumor area for the PEG-DOX conjugate, while a 29.27% reduction for hydrogel release media and 51.9% for the DOX. Both PEG-DOX and the release media were internalized into A549 cells, causing cell death. The present strategy can be employed for long-term sustained delivery of chemotherapeutic agents to locally accessible tumors or sites adjacent to tumor.