The Isar is a river in Bavaria, Germany. It is a tributary of the Danube and 283 km in length. The name Isar appears to be derived from the Celtic word Isaria (meaning torrential).

The source is located in Austria not far from the German border in the Karwendel (a part of the Alps), near the town Scharnitz . At Mittenwald the river crosses into Germany. From there the river runs northwards, directly through lake Sylvenstein (a lake created for working a hydroelectric power plant, Lenggries , Bad Tölz , Geretsried , Wolfratshausen , the Bavarian capital Munich and northeastward through Freising, Landshut, Moosburg , Landau/Isar and Plattling to Deggendorf , where it finally meets the Danube River.

Important tributaries are the Jachen , the Loisach and the Amper rivers. Much of the Isar's water is drained away from it at Oberföhring (just where the river Isar is leaving Munich) by way of the Mittlere Isar Kanal, a canal that directs the water of the Isar to the artificial lake Speichersee where another hydroelectric power plant generates electricity. From there the canal is headed north, and joins the River Isar northeast of Moosburg .

Floodings are much less likely since the 1950s when Sylvenstein Dam in the 1950s south of Lenggries in order to generate electicity. There have been high water levels in the river Isar since then (e. g. in 1979 and 1999 ) but no actual floodings occurred.