Sønderborg (sö`nərbôr), Ger. Sonderburg, city (1992 pop. 25,551), Sønderjylland co., S Denmark, on both sides of the Als Sund. The older section of the city is situated on Als Island and is connected by bridge with the newer quarter on Jylland. Sønderborg is a port, an industrial and commercial center, and a seaside resort. Manufactures include textiles, machinery, furniture, chemicals, and beer. The city developed around a 13th-century castle and was part of the duchy of Schleswig. After his deposition, Christian II was held prisoner there from 1532 to 1549. Christian III (reigned 1534–59) created the duchy of Sønderborg for his younger son John, from which the branch of Schleswig-Holstein-Sønderborg-Glücksburg (which has occupied the Danish throne since 1863) is descended. The city was held by Prussia from 1864 to 1920.