The Sakaisuji Line (堺筋線, Sakaisuji-sen) is an underground rapid transit line in Osaka, Japan, operated by the Osaka Municipal Subway. Its official name is Rapid Electric Tramway Line No. 6 (高速電気軌道第6号線), while the Osaka Municipal Transportation Bureau refers to it as Osaka City Rapid Railway Line No. 6 (大阪市高速鉄道第6号線), and in MLIT publications, it is written as Line No. 6 (Sakaisuji Line) (6号線（堺筋線）).