The Marton–New Plymouth line (MNPL) is a secondary railway in the North Island of New Zealand that links the Taranaki and Manawatū-Whanganui regions. It branches from the North Island Main Trunk railway (NIMT) at Marton and runs near the South Taranaki Bight of the west coast before turning inland, meeting the Stratford–Okahukura Line (SOL) at Stratford and running to New Plymouth. Construction of the line was completed in 1885, and along with the SOL it provided an alternate route to the NIMT with the SOL's completion in 1933 until the latter was mothballed in 2010. In its early days, the North Island's first regional express, the New Plymouth Express ran on the line, but it has been freight-only since the cancellation of passenger services on the line in 1977.

Marton is a town in the Rangitikei district of the Manawatū-Whanganui region of New Zealand's North Island. It is situated 35 kilometres southeast of Whanganui and 40 kilometres northwest of Palmerston North.