Control of gonadotropin-releasing hormone (GnRH) signalling is an effective strategy for the treatment of sex hormone-dependent diseases. GnRH analogues have been widely used for treating these diseases; however, initial stimulation or complete suppression of GnRH signalling by GnRH analogues results in the occurrence of several distinct adverse effects. Accordingly, we aimed to discover small molecule GnRH antagonists with superior pharmacokinetic and pharmacodynamic profiles. Linzagolix is a potent, orally available, and selective GnRH antagonist. Here, we reported the pharmacological characterization of linzagolix in vitro and in vivo. Linzagolix selectively binds to the GnRH receptor and inhibits GnRH-stimulated signalling, in a manner comparable to cetrorelix, a peptide GnRH antagonist. Because the inhibitory effect of the gonad axis is useful for the treatment of gynaecological conditions such as endometriosis and uterine fibroids, we investigated the effect of orally administrated linzagolix on the gonadal axis in ovariectomized and intact cynomolgus monkeys. In ovariectomized monkeys, linzagolix immediately suppressed the serum luteinizing hormone concentration at doses over 1 mg/kg, indicating dose-dependent inhibition that correlated with serum linzagolix concentrations. In intact female monkeys, repeated linzagolix administration suppressed hormone surges and ceased or prolonged menstrual cycles. Furthermore, all animals presenting arrested menstrual cycles following linzagolix treatment showed recovery of hormone secretion and regular menstrual cycles after administration periods ended. Our results demonstrated that linzagolix has potential as a novel agent for reproductive-age women suffering from sex hormone-dependent diseases.