A robber baron or robber knight (German: Raubritter) was an unscrupulous feudal landowner who, protected by his fief's legal status, imposed high taxes and tolls out of keeping with the norm without authorization by some higher authority. Some resorted to actual banditry. The German term for robber barons, Raubritter (robber knights), was coined by Friedrich Bottschalk in 1810.

Some robber barons violated the custom under which tolls were collected on the Rhine either by charging higher tolls than the standard or by operating without authority from the Holy Roman Emperor altogether. During the period in the history of the Holy Roman Empire known as the Great Interregnum (1250–1273), the number of such tolling stations exploded in the absence of Imperial authority.

Medieval robber barons most often imposed high or unauthorized tolls on rivers or roads passing through their territory. Some robbed merchants, land travelers, and river traffic—seizing money, cargoes, entire ships, or engaged in kidnapping for ransom.