Beaufort or Belfort Castle, known locally as Qal'at al-Shaqif (Arabic: قلعة الشقيف, romanized: Qalʿat al-Shaqīf) or Shaqif Arnun (شقيف أرنون, Shaqīf Arnūn), is a Crusader fortress in Nabatieh Governorate, Southern Lebanon, about 1 kilometre (0.62 mi) to the south-south-east of the village of Arnoun. There was a fortification on the site before it was captured by Fulk, King of Jerusalem, in 1139 and construction of the Crusader castle probably began soon after. Saladin captured Beaufort in 1190, but 60 years later Crusaders re-took it. In 1268 Sultan Baibars finally captured the castle for the Islamic forces.

Beaufort provides one of the few cases in which a medieval castle proved of military value and utility in modern warfare as well, as shown by its late 20th-century history, especially during the 1982 Lebanon War.

The castle was named bel fort or beau fort (Old French for 'beautiful fortress') by the Crusaders who occupied the castle in the 12th century. Its Arabic name Qalʿat al-Shaqīf means 'Castle of the High Rock', with shqif being a borrowing from Aramaic.^{[citation needed]}

The outcrop Beaufort occupied overlooks the Litani River. The river flows past the east side of the castle, which stands atop a 300 metres (980 ft) cliff which declines steeply to the river. Little is known of the site prior to its capture by Crusader forces in 1139, as no contemporary documents mention the site before then. However, historians assume that the castle's commanding hilltop site made it a strategic position that was fortified before its capture by the Crusaders. Fulk, King of Jerusalem, captured the fortification of Qal'at al-Shaqif in 1139 and gave the site to the lords of Sidon. Medieval historian Hugh Kennedy speculates that construction of the Crusader castle began soon after Fulk gave the site to the lords of Sidon.

Saladin defeated the Crusaders at the Battle of Hattin in 1187. In the aftermath, many castles and cities fell to Saladin's forces so that only a handful of cities remained under the Crusaders' control. Beaufort was one of the last castles to resist Saladin. In April 1189, Saladin was preparing to besiege the castle and Arab sources describe the event in detail. At the time Beaufort was under the control of Reynald of Sidon who had survived the Battle of Hattin. While Saladin was camped at nearby Marjayoun, preparing for the siege, Reynald met him and claimed to have Muslim sympathies. He said that while he would like to hand over control of Beaufort, his family were in the Christian city of Tyre and he could not surrender until they were safely out of the city. In the hope of a taking the castle without any bloodshed, Reynald was given three months to extract his family from Tyre; instead he used this time to repair the castle and stock up on supplies.

After three months Reynald met Saladin again, protesting he needed more time. Saladin insisted he hand over the castle immediately, so Reynald ordered the garrison to surrender. When they refused Reynald was taken prisoner and the siege began. Hostilities lasted until August that year when Saladin was forced to lift the siege to defend Acre. In April 1190 an agreement was reached where the castle's garrison would hand over control to Saladin in return for Reynald's release. The castle came under Crusader control in 1240 as part of a treaty negotiated by Theobald I of Navarre. It was sold to the Knights Templar by Reginald's grandson, Julian of Sidon, in 1260. In 1268, the Mamluke Sultan Baibars captured the castle, and there was relative calm through the 14th, 15th, and 16th centuries.

After the Ottoman conquest of Syria in 1516, the Ottomans attempted to revive the area by granting military benefices (timar) to Ottoman cavalry soldiers around Shaqif Arnun castle. The Shiite Sa'b family held the castles on the Ottomans behalf as early as 1571. In the early 17th century Fakhr-al-Din II took the castle as a part of his network of fortifications. Fakhr-al-Din II was defeated by the Ottomans, who destroyed the upper portions of the castle. Thereafter it was reinvested by the Sa'bs. The area was ruled by Shiite feudal families until 1769. In 1782 the Governor of Acre, Jazzar Pasha, besieged the castle, captured it and destroyed many of its remaining fortifications. The Galilee earthquake of 1837 caused further damage to the structure and from then on the ruins were used as a quarry and a shelter for sheep. The late 19th century saw the start of study of Beaufort Castle, with surveys by Victor Guérin in 1880 and Claude Reignier Conder and Herbert Kitchener in 1881 as part of the Survey of Western Palestine.

T. E. Lawrence visited the castle in 1909 during his walk across modern-day Lebanon and Syria, conducting research for his thesis. He was particularly struck by the view of the coast and along the Litani River.