Odra or Oder, river in north central Europe, an important transportation artery. It rises near Olomouc in the Czech Republic and flows generally northeast into Poland, where it is the second longest river. The Odra continues generally north past Racibórz, Opole, Wrocław, and Kostrzyn into the Baltic Sea at Szczecin. The river is 912 km (567 mi) long and navigable from Racibórz. The Odra is connected by canal with the Spree, Havel, and Elbe rivers in eastern Germany.

Before World War II the Odra flowed through German territory; with the defeat of Germany in 1945 a provisional German-Polish border was set along a line formed by the Odra and Neisse rivers. The government of West Germany delayed recognizing the new border and the subsequent loss of about 103,600 sq km (about 40,000 sq mi) of land until 1970. In 1990 West and East Germany united and became the Federal Republic of Germany, and reaffirmed the 1945 border.