The Kotoden Nagao Line (琴電長尾線, Kotoden Nagao-sen) is a Japanese railway line in Kagawa Prefecture, which connects Kawaramachi Station in Takamatsu with Nagao Station in Sanuki.

It is owned and operated by the Takamatsu-Kotohira Electric Railroad.

All stations are located in Kagawa Prefecture.

The line first opened as the Takamatsu Electric Tramway (高松電気軌道, Takamatsu Denki Kidō) on 30 April 1912 between Dehare (出晴) (close to the present Kawaramachi Station) and Nagao.

The line was originally 1,067 mm (3 ft 6 in) gauge and electrified at 600 V DC, but it was regauged to 1,435 mm (4 ft 8+1⁄2 in) in June 1945, and the overhead line voltage was raised to 1,500 V from December 1976.