Ostallgäu (German pronunciation: [ˈɔstˌʔalɡɔʏ]) is a Landkreis (district) in Swabia, Bavaria, Germany. It is bounded by (from the west and clockwise) the districts of Oberallgäu, Unterallgäu, Augsburg , Landsberg, Weilheim-Schongau and Garmisch-Partenkirchen, and by the Austrian state of Tyrol. The town of Kaufbeuren is enclosed by but does not belong to the district.

Before 1803 the region was split into several tiny states, most of them clerical states. When these states were dissolved in 1803, the Ostallgäu region became part of Bavaria. The kings of Bavaria soon developed a special relationship with the region and built their famous castles of Hohenschwangau and Neuschwanstein there.