Hamrštejn, Gothic castle ruins in Machnín, Czech Republic.

Hamrštejn is a castle ruin in Machnín with Gothic architecture, its remains comprising two towers and sections of outer wall. The structures sit on a ridge overlooking the Lusatian Neisse river valley.

Hamrštejn was first documented in 1357, when Bedřich of Bieberstein pledged loyalty to Charles IV; it served as the administrative centre of the southwestern part of the Bieberstein estate based at Frýdlant. In 1433 Hussite forces under Jan Čapek of Sány captured it, and it remained in Hussite hands until 1435. By 1558 it was recorded as deserted.

The castle's name is linked to one of Bohemia's oldest hammer mills, an ironworks that disappeared by the end of the 16th century; its exact site is unknown. The first known burgrave, Hans Dachs, is recorded in 1409, and a garrison of several dozen men served under him.

The ruin is reachable by train to Machnín hrad station or via the Odra-Neisse cycle path number 20. Visitors should wear sturdy shoes, as the terrain is uneven and partly steep.

The ferry near the castle is Czechia's only transbordér, a cable ferry that carries passengers and cyclists across the Lusatian Neisse. It is operated by hand and connects the railway stop at Machnín hrad with the opposite bank.