Glaucoma is second leading cause of blindness worldwide which is characterized by progressive degeneration of retinal ganglion cells (RGCs). Elevated intraocular pressure (IOP) is one of the highest risk factors and IOP-lowering agents are used to prevent glaucoma. New molecular target is required because of the side effects, drug resistance, and insufficiency for IOP reduction by a part of pre-existing agents. Here, we report that P2Y₁ receptor (P2Y₁R) activation induces IOP reduction and knock out of P2Y₁R (P2Y₁KO) causes sustained IOP elevation associated with age-dependent RGC degeneration. Topical application of MRS2365, selective agonist for P2Y₁R, caused significant reduction in IOP in wild-type (WT) mice but not in P2Y₁KO mice. We also found that P2Y₁KO mice showed significantly higher IOP level than that in WT mice. Because sustained IOP elevation is one feature of hypertensive glaucoma, we checked RCG damages and found that the number of RGCs in P2Y₁KO mice was comparable at 3 months old but significantly smaller at 12 months old. Furthermore, optical coherence tomography (OCT) revealed that 12-month-old P2Y₁KO mice showed thinner ganglion cell and inner plexiform layers, general diagnostic feature of glaucoma patients. Taken together, our results demonstrated that (1) P2Y₁R activation reduces IOP; (2) loss-of-function of P2Y₁R causes sustained elevation in IOP and (3) hypertensive glaucoma-like phenotypes in middle-aged mice.