Santiago Masarnau y Fernández (also known as Santiago Fernández de Masarnau or Santiago [de] Masarnau) (9 December 1805 in Madrid – 14 December 1882 in Madrid) was a Spanish pianist, composer and religious activist for the poor. He established the Society of Saint Vincent de Paul, an organization composed of laymen dedicated to serving the poor, in Spain. A cause for his canonization has been opened by that society.
Masarnau was born in Madrid 1805 to Santiago Masarnau y Torres, a native of Copons in Catalonia and Beatriz Fernández y Carredano, from the Omoño sector of the town of Ribamontán al Monte in Cantabria, a family with close connections to the Spanish aristocracy and court. The mother died in 1808. At that same time, after the abdication of King Ferdinand VII of Spain and the subsequent outbreak of the Peninsular War, Masarnau senior was appointed the Secretary of the , in the service of the Count of Miranda, and the father moved with his three children to Andalucia. The son proved to be a musical prodigy in his childhood, and soon began the study of music under the organist of the Cathedral of Granada, José Rouré y Llamas. When King Ferdinand regained the throne in 1814, the father was appointed a secretary to the Chief Royal Majordomo (Camarero mayor) and the family returned to Madrid. There the son was able to participate in the musical life of the Escorial, performing on the organ before King Ferdinand (including some of his own compositions) when he was only ten years old. Life at court was highly dependent on royal favour, and Masarnau's father was obliged, for unclear reasons, to quit his privileges as a Gentleman of the Royal Household in the 1820s.