The Yokoyama Party, centered in the Yokoyama-sho estate in Tama County, Musashi Province (the present Hachioji City, Tokyo), was an armed group from the same family that existed from the late Heian period to the Kamakura period, and based themselves in Musashi Province (Osato County, Hiki County, and Tachibana County) and the northern part of Sagami Province.
横山党（よこやまとう）は、平安時代後期から鎌倉時代にかけて、武蔵国多摩郡（現・東京都八王子市）横山庄を中心として、武蔵国（大里郡・比企郡-橘樹郡）および相模国北部に割拠した同族的武士団である。