Als (German Alsen) is an island in the Baltic sea, off the coast of Schleswig, in the Little Belt. It belonges to Denmark, but, as a result of the Danish war of 1864, it became German until 1920.

• Area: 321 km2
• Inhabitants: ca. 60.000

Sonderburg, the capital, with a good harbour and a considerable trade, is connected with the mainland by a pontoon bridge. Other places of note are Norburg and Augustenburg. On the peninsula Kekenis at the south west end of Alsen there is a lighthouse. Here, in 1848, the Danes directed their main attack against Fieldmarshal Wrangel's army. In 1864 the Prussians under Herwarth von Bittenfeld took Alsen, which was occupied by 9000 Danish troops, thus bringing the Danish war to a close. Since 1870 Alsen has been fortified.

See also: List of islands of Denmark