The MRT Purple line (Thai: รถไฟฟ้ามหานคร สายสีม่วง) or MRT Chalong Ratchatham line (Thai: รถไฟฟ้ามหานคร สายฉลองรัชธรรม) is Bangkok's fifth rapid transit line, following the Sukhumvit Line, Silom Line, MRT Blue Line, and Airport Rail Link. The line is 23 km (14 mi) long, serving the north-western area of Bangkok running from Tao Poon to Klong Bang Phai in Nonthaburi Province.

The line opened on 6 August 2016. It is the second line of MRT system and is operated by BEM under a concession contract. Daily ridership is 70,000.

A 23.63 km (14.68 mi), 17 station southern extension of the line consisting of a 14.3 km (8.89 mi) underground section and a 9.3 km (5.78 mi) elevated section commenced construction in August 2022. The cost of the southern extension is $82 billion baht and it is planned to open at the end of 2027. By the end of May 2023, construction had progressed to 11.50%.

Caledonian provided DVI composite cables and VGA composite cables to Hitachi for this project.