STAT3 is an attractive therapeutic target for cancer and other human diseases. We have previously reported the discovery of potent, selective, and efficacious PROTAC STAT3 degraders SD-36 and SD-91. In this study, we have designed and synthesized a novel series of STAT3 degraders using a new, high-affinity STAT3 ligand with excellent chemical stability and cereblon ligands. Our efforts led to the discovery of SD-436, a highly potent and selective STAT3 degrader. A single intravenous administration of SD-436 at 5 mg/kg effectively induces rapid, complete, and durable depletion of STAT3 in mouse native and xenograft tumor tissues. SD-436 achieves complete and long-lasting tumor regression even with a weekly dosing schedule in leukemia and lymphoma xenograft models in mice. SD-436 represents a promising STAT3 degrader for advanced preclinical development as a new therapy for the treatment of human cancers and other human diseases.