Juan Bautista Martínez del Mazo (c.1612 – February 10, 1667) was a Spanish Baroque portrait and landscape painter, the most distinguished of the followers of his father-in-law Velázquez, whose style he imitated more closely than did any other artist.

Del Mazo married the famous painter's only surviving daughter, Francisca de Silva Velázquez y Pacheco, on August 21, 1633, at the Church of Santiago in Madrid.

In 1643, Mazo became Master of Drawing and personal painter of the heir to the Spanish Crown, Baltasar Carlos, Prince of Asturias, who in 1645 became godfather of his fifth child.

Prince Baltasar Carlos commissioned him to copy hunt scenes by Paul de Vos, Rubens, Jordaens and other Flemish painters.

Mazo first expressed his talent copying works of Venetian masters in the royal collections, such as Tintoretto, Titian, and Paul Veronese, a task that he performed with assiduity and success.

In fact, there are few extant paintings that scholars agree are his; these include View of Saragossa (1646, Prado); Portrait of Queen Mariana in Mourning Dress (1666, National Gallery, London); and The Family of the Painter (c. 1660–1665, Kunsthistorisches Museum, Vienna).

Although Mazo acquired great skills as a portrait painter, some of his most interesting works are hunting pieces and landscapes in which he developed a more personal style with a respect for reality.

[1] The departure from his master's style is reflected in his way of shaping people and things by highlights which flash the pictorial image towards the surface of the painting, even from the background.

[1] A further departure from Velázquez is his luxurious depiction of detail or incident which he achieved with brilliant, depthless strokes, whether on the figure of a sitter, a curtain on a wall, a floor, the surface of a river, or on plain earthen grounds.

After Velázquez's death in 1660, Philip IV appointed him as the official Court painter on April 19, 1661, while his son Gaspar took his former position as Usher to the Chamber.

At the death of Philip IV in 1665, Mazo kept his position at Court under the regency of Queen Mariana, whose portrait in mourning dress (National Gallery, London, 1666) is one of his few signed works.