Olza (help·info) (Czech: Olše, German: Olsa) is a river in Poland and the Czech Republic, the right tributary of the Oder River. It flows from the Silesian Beskids through southern Cieszyn Silesia in Poland and Frýdek-Místek and Karviná districts of the Czech Republic, often forming the border with Poland. It flows into the Oder River north of Bohumín. The Olza-Oder confluence also forms a border.

It is a symbol of the Zaolzie (Polish: Trans-Olza River) region, which lies on its west bank, constituting a part of the western half of Cieszyn Silesia. The unofficial anthem of this region and local Poles, Płyniesz Olzo po dolinie (You flow Olza, down the valley), written by Jan Kubisz, is centered around the river.

Olza inspired many other artists. Writers who wrote about the river include Adolf Fierla, Pola Gojawiczyńska, Emanuel Grim, Julian Przyboś, Vladislav Vančura, Adam Wawrosz. Singer Jaromír Nohavica used the motive of Olza in several of his songs.