Lake Urmia (Persian: دریاچهٔ ارومیه) is a salt lake in northwestern Iran, in Iranian Azarbaijan (between the provinces of East Azarbaijan and West Azarbaijan), west of the Caspian Sea. It is the largest lake inside Iran, with a surface area of approximately 5,200 km² (2,000 mile²). At its maximum extent, it is about 140 km (87 miles) long, and 55 km (34 miles) wide. Its deepest point is approximately 16 m (52 ft) deep.

The lake is named after the city Urmia, an originally Syriac name meaning city of water. It was called Lake Rezaiyeh (دریاچهٔ رضائیه) in the early 1930s after Reza Shah Pahlavi, but the lake once again was named Urmia in the mid-1970s.

The lake is marked by more than a hundred small rocky islands, which are stopover points in the migrations of various kinds of wild bird life (including flamingos, pelicans, spoonbills, ibises, storks, shelducks, avocets, stilts, and gulls). The second largest island, Kaboudi , is the burial place of Hulagu Khan, the grandson of Genghis Khan and the sacker of Baghdad, where he had his treasury.

The lake itself is too salty for living fish. Most of the area of the lake is considered a national park.

The lake is a major barrier between two of the most important cities in Iranian Azerbaijan, Urmia and Tabriz. A project to build a bridge across the lake was started in the 1970s but was abandoned after the Iranian Revolution of 1979. The project was revived in the early 2000s, and is due to finish in 2004 or 2005.

Lake Urmia has been shrinking for a long time, with an annual evaporation rate of 0.6 to 1 m (24 to 39 inches). The lake's salts are considered to have medical effects, especially as a cure for rheumatism.