Morské oko (called Veľké Vihorlatské jazero in the past; literally "Eye of the sea") is a lake in the Vihorlat Mountains in eastern Slovakia. It is the largest non karst lake and the third biggest natural lake in Slovakia. It is at an altitude of 618 m, covers 0.13 km² with a maximum depth of 25.1 m. It is drained by the river Okna.

The oldest known description of the lake can be found on two maps, dating back to 1687. The earliest official name of the lake known as Blatné jazero (germ. Blatto teich) dates back to 1784. The name Veľké Vihorlatské jazero was introduced in 1933.

The lake itself is a natural home to 8 species of fish. The native species consist of the brown trout (Salmo trutta m. fario), common minnow (Phoxinus phoxinus) and stone loach (Barbatula barbatula). In the past, rainbow trout (Oncorhynchus mykiss) was introduced into the lake, but the prevailing species today is the common chub (Squalius cephalus).

It is a national nature reserve (covering 1.08 km²) since 1984 and part of the Vihorlat Protected Landscape Area.

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  Mansion Morské oko, built in 1924 for Gladys Vanderbilt Széchenyi
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  View of Morské oko from Sninský kameň in spring

• Protected areas of Slovakia

• National nature reserve Morské Oko at the State Inventory of Specially Protected Parts of Nature and Landscape
• Morské oko - Sirava.net (in Slovak)
• Morské oko at Sirava.sk (many pictures) (in Slovak)