Optymistychna (Ukrainian: Оптимістична; meaning "optimistic") is a gypsum cave located near the Ukrainian village of Korolivka, Chortkiv Raion, Ternopil Oblast. Approximately 264 km (164 mi) of passageways have been mapped within. As a result, it is the longest cave in Europe and the seventh-longest cave in the world, after Mammoth Cave, Sistema Sac Actun, Jewel Cave, Shuanghedong and Sistema Ox Bel Ha. It is also the longest gypsum cave in the world.

The cave complex was discovered by the speleologists of the Lviv speleological club "Cyclope" in 1966. It was entirely unknown before then. There have been more than 50 expeditions since then, but exploration has slowed significantly in recent years, and very little surveying is currently being done. The cave is located very close to the Priest's Grotto or Ozerna Cave, the eleventh-longest cave in the world at 130.4 km (81.0 mi), but the two caves have not yet been found to be connected.

In 2008, the cave was recognized as a Natural Wonder of Ukraine.

The entire cave lies under a 2 km square area in a layer of Neogene period gypsum that is less than 30 metres (98 ft) thick. The passages tend to be fairly small, no more than 3 metres (10 ft) wide and 1.5 metres (5 ft) tall for most, although at intersections they can be up to 10 metres (33 ft) tall. They are often choked with mud. They comprise a dense network on several levels, making Optymistychna known as a "maze cave".

Optymistychna's gypsum bed is topped with a limestone layer, which has seeped through into the cave via erosion and formed into calcite speleothems. At other places, the gypsum has formed crystals, often tinted a multitude of colors by mineral salts. In some areas, large gypsum rosettes have formed, colored black by manganese oxide.

• List of longest caves

• Official site of Optimistic cave
• Chapter II.9, Gypsum Karst in the Western Ukraine by Alexander Klimchouk article appearing in: International Journal of Speleology, vol. 25, #3-4, 1996, p. 263-278.