Juan Blas de Castro (1561 – August 6, 1631) was a Spanish singer, musician and composer.
In 1597, Castro became a court musician to King Philip III of Spain, and, from June 15, 1599 an usher.
Back in Madrid from Valladolid, from 1606 he would collaborate closely with Lope de Vega, who would praise his art.
In the Cancionero de la Sablonara, eighteen of his tonos survived.
Lope and Góngora were the masters of the tono, romances with profane refrains.