The siege of Tripoli occurred in 1551 when the Ottoman Turks and Barbary pirates besieged and vanquished the Knights of Malta in the Red Castle of Tripoli, modern Libya. The Spanish had established an outpost in Tripoli in 1510, and Charles V remitted it to the Knights in 1530. The siege culminated in a six-day bombardment and the surrender of the city on 15 August.

The Red Castle, in Arabic As-saraya Al-hamra, sometimes also Red Fort or Red Saraya, is a major landmark on the waterfront of Tripoli, bordering Martyrs' Square. It has been the home of the Red Castle Museum since 1919, and of the Libyan Department of Archaeology since 1952.