The Syvash or Sivash, also known as the Putrid Sea or Rotten Sea (Russian: Гнило́е Мо́ре, Gniloye More; Ukrainian: Гниле́ Мо́ре, Hnyle More; Crimean Tatar: Çürük Deñiz, is a large area of shallow lagoons on the west coast of the Sea of Azov. Separated from the sea by the narrow Arabat Spit, the water of the Syvash covers an area of around 2,560 km2 and the entire area spreads over about 10,000 km2. The Henichesk Strait is its eastern connection to the Sea of Azov. The Syvash borders the northeastern coast of the main Crimean Peninsula. Central and Eastern Syvash were registered as wetlands of Ukraine under the Ramsar Convention. Since the 2022 Russian invasion of Ukraine, the entire Syvash has been occupied by Russia.

The Sea of Azov is an inland shelf sea in Eastern Europe connected to the Black Sea by the narrow Strait of Kerch, and is sometimes regarded as a northern extension of the Black Sea. The sea is bounded by Russia on the east, and by Ukraine on the northwest and southwest, currently under Russian occupation. It is an important access route for Central Asia, from the Caspian Sea via the Volga–Don Canal.