Stahleck Castle (German: Burg Stahleck) is a 12th-century fortified castle in the Upper Middle Rhine Valley at Bacharach in Rhineland-Palatinate, Germany. It stands on a crag approximately 160 metres (520 ft) above sea level on the left bank of the river at the mouth of the Steeg valley, approximately 50 kilometres (31 mi) south of Koblenz, and offers a commanding view of the Lorelei valley. Its name means "impregnable castle on a crag", from the Middle High German words stahel (steel) and ecke (here: crag). It has a water-filled partial moat, a rarity in Germany. Built on the orders of the Archbishop of Cologne, it was destroyed in the late 17th century but rebuilt in the 20th and is now a hostel.

From about 1000 AD, Bacharach is presumed to have been a possession of the Archbishops of Cologne. They had the castle built, perhaps as a southern outpost to guard the far-flung archbishopric; their Vogt resided there. It is uncertain when the present castle was built to replace an earlier fortification; perhaps around 1135. It was the first large castle north of Bingen and Rüdesheim.

A "Gozwin von Stalecke" is first mentioned in charters in 1120–21. This was Goswin von Falkenburg, who is also referred to in 1135 by the Latin translation "Cozwinus de Staelechae" and was the first attested holder of the castle in fief. He was a member of a Main-Frankish family and had come into possession of the castle through marriage to Luitgard von Hengebach, the widow of Heinrich I of Katzenelnbogen, who died in 1102. From then on he called himself Goswin von Stahleck.

In 1125, Goswin's son Hermann married Gertrud of Swabia, sister of King Conrad III of Germany. After receiving Stahleck Castle in fief from his brother in law in 1140, he was additionally granted the County of Palatinate by Rhine. This made him one of the greatest lords of the Holy Roman Empire and the Four Valley Region, which consisted of the settlements of Bacharach, Steeg (now part of Bacharach), Diebach and Manubach plus the castles of Stahleck, Fürstenberg and Stahlberg, and made Stahleck the centre of power of the heart of what later became the Rhineland territory of the Counts Palatine. They developed Bacharach into a trading town, among other things for the wine trade, and the castle was thus used to enforce payment of duties. When he went on the Second Crusade, Conrad made Hermann his regent.

After Hermann's death in 1156, Emperor Frederick Barbarossa bestowed the title of Count Palatine on his half-brother Conrad of Hohenstaufen. After the death of his remaining son in 1189, Conrad's only heir was his daughter Agnes. So that the castle and the title could remain in the family, that year King Henry VI commuted the fief from a personal to a hereditary one. In the future this led to repeated conflicts between the Counts Palatine and the See of Cologne, since Stahleck was a possession of the Electoral Archbishop and not the king's to modify.

It was her father's wish for Agnes to marry King Philip II of France, but instead when Conrad was away from the castle in January or February 1194, she secretly married Henry the Elder of Brunswick, son of his enemy the Welf Duke Henry the Lion, to whom she had previously been engaged; the marriage, performed by Johann I, Archbishop of Triers, is known as the Stahleck Marriage or the Bacharach Marriage. After Conrad's death, Henry succeeded him as Count Palatine and so came into possession of the castle. However, in 1212 he renounced the title and the rights associated with it in favour of his son Henry the Younger. When the latter died young and childless in 1214, his younger sister Agnes inherited Stahleck. The non-allodial remainder of the County reverted to the Crown and was subsequently bestowed on the Bavarian Duke Ludwig I of the House of Wittelsbach.

In order to secure possession also of the castle and the overseership of Bacharach, Ludwig I arranged a marriage in 1222 between his son Otto and Agnes. Under Otto (Otto II) in 1243, the long-running conflict with the Archbishopric of Cologne was settled, and he received Fürstenberg and Stahlberg Castles as additional fiefs.

Ludwig I transferred his primary residence to Heidelberg, so that from then on, Stahleck was used only occasionally and was overseen by counts, who called themselves "Ritter (Knight) von Stahleck". From the 15th century on, it was administered by bailiffs. However, although the castle was no longer the administrative centre of the Palatinate, important gatherings of the nobility continued to take place there into the 15th century, including the election of Ludwig IV as King of Germany in May 1314 and the wedding of Emperor Charles IV and Anna, only daughter of Rudolf II, Count Palatine, on 4 March 1349. In addition, the castle was used several times during the 14th century as a pledge. In December 1314, to cover the costs of his election, Ludwig IV pledged it for 58,300 pounds of Hellers to John of Luxembourg, King of Bohemia, and his uncle Baldwin, Electoral Archbishop of Triers. In July 1328, they were required to surrender Stahleck and also Stahlberg and Braunshorn Castles as security for a fine payable to Countess Loretta of Sponheim. The castle was thus administered by Count Wilhelm I of Katzenelnbogen as regent until the entire sum of 11,000 pounds had been paid. The castle was also opened for military use to Gerlach of Nassau, Archbishop of Mainz, in 1346.