The River Mert (); anciently, the Lycastus ()is a river in Turkey which flows into the Black Sea at Samsun.
In 2022 flow measurements were lacking.
The ancient town of Lycastus in Pontus was located on the river; the site has not been located.
A local politician claims the river suffers from domestic and industrial pollution, and fish die off in 2014 is being investigated by local officials.
In 2012 flooding killed 5 people.