Lake Shamsutdin, Protected lake and natural monument in Birsky District, Russia.

Lake Shamsutdin is an oxbow of the Belaya River in the left-bank floodplain, 1.5 km west of Birsk in northern Birsky District, Bashkortostan. It stretches 8.3 km southwest to northeast, 150–400 m wide, with an average depth of 6–7 m and a maximum of 11.8 m; its surface covers 1.52 km² at 75 m above sea level. A channel on the north shore drains into the Belaya during floods, and a channel on the south shore brings constant inflow from Lake Bakakul.

Lake Shamsutdin was designated a natural monument of the Bashkir ASSR on 17 August 1965, with a protected area of 320 hectares. In 2007 traces of an ancient settlement were found on the prominent cape of the western shore, occupied by the UGAES "Ivushka" holiday base; it was named "Shamsutdin-1" and given archaeological monument status.

The name comes from the Bashkir anthroponym Shämsetdin, also rendered Shamsutdin or Shamshagin. Until 10 September 2007 the settlement on the shore was called the VTsSPS Holiday Home village. Birsk residents come for amateur fishing and recreation; the lake is also a recreational site of the protected area.

The lake lies 1.5 km west of Birsk, on the left-bank floodplain of the Belaya, within Kuseekeyevsky selsoviet and the town's territory. The western shore holds the Ivushka holiday base of UGAES, and several sanatorium-type establishments stand on the banks. The water is fed from Lake Bakakul through a channel on the south shore, and during high water it drains into the Belaya through a channel on the north shore.

The name comes from the Bashkir anthroponym Shämsetdin; the Russian form Shamsutdin and the variant Shamshagin also appear in records. On the well-defined cape of the western shore, occupied by the Ivushka holiday base, traces of an ancient settlement were found in 2007 and named Shamsutdin-1, an archaeological monument.