The conversion to a museum has respected the original interiors, which feature elaborate baroque painted ceilings commissioned by Galdiano, As he died in 1947, Galdiano bequeathed his entire personal collection of over 12,000 items to the Spanish State.

Major categories include jewellery, small bronzes, both ecclesiastical and domestic silver, ceramics, carved ivory, and numismatics.

Objects come not only from Iberia but also from major centers of medieval artistry, including Limoges and Egypt.

While the Renaissance is especially well represented, the collection features important early medieval objects, including Visigothic work, and works crafted by Iberia's ancient Celtic culture.

The painting collection includes work by artists such as George Romney, John Constable (Landscape of Flatford), Nicolaes Maes (one female portrait), David Teniers the Younger, Erasmus Quellinus II (The Virgin and the Child), Giulio Clovio (one miniature on vellum), Bernardo Cavallino (St. Stephen), Lorenzo Tiepolo, Joshua Reynolds, Francisco Zurbarán, Lucas Cranach the Elder, Hieronymus Bosch, Diego de Velázquez, Francisco de Goya, Bartolomé Esteban Murillo, Luis Paret, El Greco and Federico de Madrazo.