Lomná (Polish: Łomna, Cieszyn Silesian: Łómno) also known as Lomňanka (Polish: Łomnianka, Cieszyn Silesian: Łómnianka) is a 17 kilometres (11 mi)-long river in Frýdek-Místek District, Moravian-Silesian Region, Czech Republic, in the historical region of Cieszyn Silesia.
It is the left tributary of the Olza River, to which it enters in Jablunkov (Jabłonków). It originates near the border with Slovakia and flows through the villages of Horní Lomná (Łomna Górna), Dolní Lomná (Łomna Dolna) and Bocanovice (Boconowice).
It was mentioned in 1592 as przi rzece Lomny. The name is derived from the words łom (clatter, rumble, susurrus) or załom (turn, bend).