A rondel dagger or roundel dagger was a type of stiff-bladed dagger in Europe in the late Middle Ages (from the 14th century onwards), used by a variety of people from merchants to knights. It was worn at the waist and might be used as a utility tool, or worn into battle or in a jousting tournament as a side arm.

The rondel dagger featured a long, slim steel blade with a round or circular hand guard and pommel. Designed for close-quarter combat, it was effective in puncturing and bursting mail armor links and penetrating weak points in plate armor. The dagger evolved from the early knightly dagger and became the standard side-arm for knights in the 15th century. It was used as a backup weapon for hand-to-hand fighting and as a tool to force surrender, as captured knights could be ransomed. The rondel dagger also gained popularity among the middle class in the 15th century. Various combat techniques involving the rondel dagger can be found in Hans Talhoffer's combat manuals from the 1440s to 1460s.