Ovarian remnant syndrome (ORS) is a condition in which functional ovaries remain in the abdominal cavity after spaying, resulting in postoperative signs of estrus. A 10-month-old Ragdoll cat exhibited signs of estrus 2 months after spraying. Subsequent imaging revealed structures suggestive of the right ovary in the abdominal cavity, which were surgically removed. The cat's preoperative anti-Müllerian hormone (AMH) level was high, but the postoperative level was low. AMH levels before and after feline ORS treatment have not yet been studied. In this case, high preoperative AMH levels indicated the presence of residual ovaries, while AMH levels below the detection limit 10 days after surgery indicated the absence of functional ovarian tissue. These results highlight that AMH measurements are valuable for diagnosing and evaluating treatment outcomes in patients with feline ORS.