Derwenlas is a hamlet in northern Powys, Wales.
Part of the historic county of Montgomeryshire (Sir Drefaldwyn) from 1536 to 1974, it lies on the Afon Dyfi and was once a port serving the market town of Machynlleth.
The narrow-gauge Corris, Machynlleth and River Dovey Tramroad (opened 1859) carried slate from the quarries around Corris and Aberllefenni to the riverside quays here, including Cei Ellis and Cei Tafarn Isa, where it was loaded into ships for onward shipment.
At around the time the tramway became the Corris Railway in the early 1860s the Aberystwyth and Welsh Coast Railway extended the standard gauge rails west of Machynlleth and on reaching Derwenlas, built the new line across the bend of the river by filling in the channel and creating a new course for the Dyfi, in the process cutting off Derwenlas' quays from the river channel and killing off the port.
The Tramroad had originally continued to Cei Ward at Morben, further downriver, but the entire section west of Machynlleth was abandoned and slate was instead trans-shipped to main line trains.
Derwenlas lies on the A487 trunk road from Machynlleth to Aberystwyth.