Usurper is a derogatory term used to describe either an illegitimate or controversial claimant to the throne in a monarchy, or a person who succeeds in establishing himself as a monarch without having inherited the throne in the normal way.

In Portugal a famous usurper king was for example Miguel II, Duke of Braganza. For the Monarchic Constitution of 1838 the king Miguel and all his descendents are perpetually excluded from succession. Now also the actual Duarte Pio of Braganza, descendent of the king Miguel is considered a usurper pretender. So all the miguelist branch is considered usurper branch from the last monarchic portuguese Constitution.