One of the world's finest art museums, built around the Spanish royal collection — deep in Velázquez, Goya and Bosch in particular.
The Prado's strength is depth rather than breadth: rather than a broad survey of world art, it holds the largest and most important collections anywhere of Diego Velázquez (including Las Meninas) and Francisco Goya, spanning his entire career from court painter to the haunting "Black Paintings." Hieronymus Bosch's The Garden of Earthly Delights is another genuine must-see.
A minimum of 2–3 hours to see the highlights without rushing; art-focused visitors can easily spend a full day. Trying to fit it into a spare hour between other plans is the most common regret we hear about.
The Prado offers free entry during specific hours in the early evening on weekdays and Sunday afternoons — check current times before you go, as they shift periodically. Expect these windows to be busier than paid hours.
If you only have time for one major museum: the Prado covers historic European painting up to the early 1900s; the Reina Sofía picks up from there, housing Picasso's Guernica and Spain's modern and contemporary collection. They're complementary, not competing.
The Prado sits on the "Paseo del Arte" alongside the Reina Sofía and Thyssen-Bornemisza museums, all within a 10-minute walk of each other, with Retiro Park just behind.