Marienburg Castle is a Gothic revival castle in Lower Saxony, Germany. It is located 15 kilometres (9.3 mi) north-west of Hildesheim, and around 20 kilometres (12 mi) south of Hanover, in Hanover district. It was a summer residence of the House of Guelph. It was once given as a birthday present by King George V of Hanover (reigned 1851–1866) to his wife, Marie of Saxe-Altenburg.
It is presently being owned by Prince Ernst August of Hanover. The castle houses the property management offices of the Royal House of Hanover. Parts of it are open to the public, such as the castle museum, the restaurant, the chapel, and can be booked as an event location for weddings, receptions, concerts etc.