The development of dual prostaglandin E2 receptors 2/4 (EP2/EP4) antagonists represents an attractive strategy for cancer immunotherapy. Herein, a series of 4,7-dihydro-5H-thieno[2,3-c]pyran derivatives with potent EP2/EP4 dual antagonism were discovered by fine-tuned structural modifications. The biphenyl side chain was found to be the key pharmacophore for the transition from EP4 antagonism to EP2/EP4 dual antagonism. The introduction of large sterically hindered segments posed challenges on obtaining EP2 potency, while having minimal impact on EP4 potency. Molecular dynamics simulations verified that the EP2 pocket is relatively narrow compared to EP4, and the key residues surrounding the EP2 pocket impose spatial restrictions on the entry of antagonists. Representative compound 29 (CZY-1068) significantly reduced PGE2-induced expression of immunosuppression-related genes in macrophages. Notably, compound 29 elicited robust antitumor efficacy in the syngeneic MC38 tumor model. Taken together, this study provides a proof-of-concept for obtaining novel potent dual EP2/EP4 antagonists based on rational structural modifications.