In this report, we describe the diagnostic findings and outcome of a uterine B cell lymphoma in a mare. A well circumscribed uterine mass with significant peripheral vascularisation was identified by transrectal ultrasonography during the reproductive examination of an 8-year-old Thoroughbred mare. Subsequent hysteroscopy revealed that the uterine mass was located intramurally and contained a protruding polyp-like structure. A diagnosis of uterine B cell lymphoma was established by histopathological examination of a hysteroscopically-obtained tissue sample. Additional diagnostics demonstrated that the uterus was a site of metastasis. Due to her deteriorating condition, extensiveness of the tumour and poor prognosis, the owner decided that the mare should be subjected to euthanasia.