Altenburg är en stad med 38 000 invånare i det tyska förbundslandet Thüringen och huvudort i distriktet A Land. Staden är känd för kortspelet skat, vilket uppfanns här. Staden växte från 1400-talet fram i skuggan av Altenburgs slott. 1826-1918 var A huvudstad i hertigdömet Sachsen-Altenburg.
A was first mentioned in 976 and later became one of the first German cities within former Slavic area, east of the Saale river (as part of the medieval Ostsiedlung movement). The emperor Frederick Barbarossa visited A several times between 1165 and 1188, hence the town is named a Barbarossa town today. Since the 17th century, A was the residence of different Ernestine duchies, of whom the Saxe-Altenburg persisted until the end of monarchy in Germany in 1918. Industrialization reached A and the region quite early in the first half of the 19th century and flourished until the Great Depression around 1930. Economic malaise set in while A was in East Germany and continued after German reunification in 1990, evidenced by a decline in population, high unemployment and house vacancy rates.
The main sights of A are the castle, the Lindenau-Museum, the historic city centre (most buildings are from early-modern origin) and the Gründerzeit architecture around the centre. The popular German card game Skat was developed in A during the 1810s and the founder of the famous Brockhaus Enzyklopädie, Friedrich Arnold Brockhaus, lived and worked in A between 1810 and 1817.
Altenburg lies in the flat and fertile landscape of Osterland on the Pleiße river in the very east of Thuringia, next to the neighboring federal state of Saxony.