Juan Blas de Castro (1561 – August 6, 1631) was a Spanish singer, musician, and composer.
Born in Barrachina, in the province of Teruel, Spain, he was the second of four brothers. In 1592, he became part of the court of the Duke of Alba in Alba de Tormes, Salamanca, together with his friend, the poet and playwright Lope de Vega. During his stay, he set to music several of Lope's poems. Both would leave Salamanca together. In 1597 Castro became a court musician to King Philip III of Spain, and, from June 15, 1599 an usher. He died in Madrid.