KOS may refer to:

• KOS Media, providing county wide and local regional publications in Kent
• Kill on sight
• Kingdom of Sorrow a Sludgecore band from New Orleans
• Knowledge Organization Systems, a generic term used in Knowledge organization
• KolibriOS, open source x86 operating system
• KOS (Yugoslavia), the counterintelligence service of the Yugoslav People's Army
• Kickoff specialist, a position in American football and Canadian football responsible for handling kickoff duties
• Kid operating system, a French software project

Kos or Cos (, ) is a Greek island, part of the Dodecanese island chain in the southeastern Aegean Sea, off the Anatolian coast of Turkey. Kos is the third largest of the Dodecanese by area, after Rhodes and Karpathos; it has a population of 33,388 (2011 census), making it the second most populous of the Dodecanese, after Rhodes. The island measures , and is from the coast of the ancient region of Caria in Turkey. Administratively, Kos constitutes a municipality within the Kos regional unit, which is part of the South Aegean region. The principal town of the island and seat of the municipality is Kos town.

Kos, meaning "blackbird", is a Slavic surname very common in Slovenia. It is the 10th most common surname in Slovenia. Unlike many other Slovene surnames, Kos is found throughout the country, although it's more common in the central and eastern regions than in the west.

The Slavic word for "blackbird", kos (kosъ, кос) also appears in various toponyms:

• Kosovo (toponym)
  • Kosovo, a partially recognized country in the Balkans.
  • Kosovo Polje