Alsen, a Baltic island in the Little Belt, in lat. 55° N., lon. 10° E. It was taken from the Danes by the Prussians, June 29, 1864, and now constitutes, together with a portion of the opposite mainland (joined by a drawbridge), the district of Sonderburg, in the Prussian province of Schleswig-Holstein; area, about 125 sq. m.; pop. in 1868, 34,551. The island is fertile, and contains fine woods and fresh-water lakes, which abound with fish.