Heidelberg Castle (German: Heidelberger Schloss) is a ruin in Germany and landmark of Heidelberg. The castle ruins are among the most important Renaissance structures north of the Alps.

The castle has only been partially rebuilt since its demolition in the 17th and 18th centuries. It is located 80 metres (260 ft) up the northern part of the Königstuhl hillside, and thereby dominates the view of the old downtown. It is served by an intermediate station on the Heidelberger Bergbahn funicular railway that runs from Heidelberg's Kornmarkt to the summit of the Königstuhl.

The earliest castle structure was built before 1214 and later expanded into two castles circa 1294; however, in 1537, a lightning bolt destroyed the upper castle. The present structures had been expanded by 1650, before damage by later wars and fires. In 1764, another lightning bolt caused a fire which destroyed some rebuilt sections. By 1880, Mark Twain mentioned it as a ruin.

Heidelberg was first mentioned in 1196 as "Heidelberch". In 1155 Conrad of Hohenstaufen was made the Count Palatine by his half-brother Frederick Barbarossa, and the region became known as the Electoral Palatinate. The claim that Conrad's main residence was on the Schlossberg (Castle Hill), known as the Jettenbühl, cannot be substantiated. The name "Jettenbühl" comes from the soothsayer Jetta, who was said to have lived there. She is also associated with Wolfsbrunnen (Wolf's Spring) and the Heidenloch (Heathens' Well). The first mention of a castle in Heidelberg (Latin: "castrum in Heidelberg cum burgo ipsius castri") is in 1214, when Louis I, Duke of Bavaria of the House of Wittelsbach received it from Hohenstaufen Emperor Friedrich II. The last mention of a single castle is in 1294. In another document from 1303, two castles are mentioned for the first time:

All that is known about the founding of the lower castle is that it took place sometime between 1294 and 1303. The oldest documented references to Heidelberg Castle are found during the 1600s:

All of these works are for the most part superficial and do not contain much information. In 1615, Merian's Topographia Palatinatus Rheni described Prince Elector Ludwig V as he "started building a new castle one hundred and more years ago". Most of the descriptions of the castle up until the 18th century are based on Merian's information. Under Ruprecht I, the court chapel was erected on the Jettenbühl.

When Ruprecht became the King of Germany in 1401, the castle was so small that on his return from his coronation, he had to camp out in the Augustinians' monastery, on the site of today's University Square. What he desired was more space for his entourage and court and to impress his guests, but also additional defences to turn the castle into a fortress.

After Ruprecht's death in 1410, his land was divided between his four sons. The Palatinate, the heart of his territories, was given to the eldest son, Ludwig III. Ludwig was the representative of the emperor and the supreme judge, and it was in this capacity that he, after the Council of Constance in 1415 and at the behest of Emperor Sigismund, held the deposed Antipope John XXIII in custody before he was taken to Burg Eichelsheim (today Mannheim-Lindenhof).