© 2015 International Society of Gynecological Pathologists. The levonorgestrel-releasing intrauterine system (L-IUS) is widely used in contraception and in the treatment of menorrhagia, dysmenorrhea, adenomyosis, and endometriosis. L-IUS is also increasingly considered in the management of endometrial neoplasia and its precursors. Histologic changes in the endometrium can be due to the effects of high-dose progestogen or may be caused by the local irritant or mechanical effects of an intrauterine foreign body. In the present study, we describe a novel endometrial alteration associated with L-IUS that most closely resembles synovial metaplasia reported at other extra-articular anatomic sites. Eleven cases were identified with a mean age of 49.6 yr. In most patients L-IUS was used for management of menorrhagia or endometrial hyperplasia. Endometrial synovial-like metaplasia was always a focal finding and was associated with areas of surface epithelial erosion. The synovial-like cells showed a distinctive palisaded arrangement with orientation perpendicular to the endometrial surface. Multinucleate cells were present in 2 cases, but granulomas were not identified. The synovial-like cells were vimentin immunoreactive and a variable proportion of cells expressed CD68. Only focal CD10 staining was seen and there was no expression of estrogen receptor, progesterone receptor, or cytokeratin. In summary, L-IUS may be associated with a distinctive synovial-like metaplastic alteration which most likely represents a stromal reaction to an intrauterine foreign body following endometrial surface erosion. The synovial-like cells appear to comprise histiocytes and modified fibroblasts or stromal cells similar to this process in other sites.