The Merkys ( Miarkis) is a river in southern Lithuania and northern Belarus. It flows for through Belarus, along the Belarusian–Lithuanian border, and through Lithuania before joining the Neman River near Merkinė.
Merkys is mostly fed by underground streams and therefore is cooler during summers and has smaller fluctuations in water level than other rivers in Lithuania. Near Žagarinė ( before its mouth) the Merkys is connected with Lake Papys by a canal. The Vokė originates from this lake and consumes most of the Merkys' water. Before the canal average discharge of the Merkys is and below it only . At the end of the 19th century the drainage basin of the Merkys grew by some as its tributary Ūla River overtook some of the Kotra's watershed area.
The Merkys is a popular destination among water tourism enthusiasts as part of it belongs to the Dzūkija National Park and it flows into the Neman near the historical site of Merkinė. Archaeological findings show that people inhabited the area as early as the Mesolithic period. The Merkys is known for its diverse fauna, being declared a reservoir for trout in 1974.