The Leitha (German: [ˈlaɪtaː] ⓘ; Hungarian: Lajta, formerly Sár(-víz); Slovene: Litva; Czech and Slovak: Litava) is a river in Austria and Hungary, a right tributary of the Danube.

The Leitha rises in Lower Austria at the confluence of its two headstreams, the Schwarza, discharging the Schneeberg, Rax and Schneealpe ranges of the Northern Limestone Alps, and the Pitten.

"[4] After the Hungarian conquest of the Carpathian Basin in the late 9th century, the Magyar horsemen dared further invasions into the adjacent West Frankish lands, until they were finally defeated by King Otto I in the 955 Battle of Lechfeld.

Thereafter the forces of the Bavarian duke Henry the Wrangler gradually re-conquered the lands beyond the Vienna Woods up to the Leitha River, where about 976 the March of Austria (Ostarrîchi) was established under the Babenberg margrave Leopold I.

Around the turn to the 2nd millennium, the Hungarian frontier (Gyepű) ran along the Leitha shore, from 1156 onwards it formed the eastern border of the Duchy of Austria with fortresses erected at Wiener Neustadt, Bruck and Hainburg.

The course of the border was confirmed in a 1411 deed issued by King Sigismund, when his daughter Elizabeth married the Habsburg duke Albert II of Austria.

Upon the dissolution of Austria-Hungary after World War I, the 1920 Treaty of Trianon adjudicated the West Hungarian territory of the proclaimed Lajtabánság (Leitha Banat) to the Republic of Austria (as Burgenland), whereby the course of the river became an inner Austrian border.