Juan Bautista Lázaro de Diego (León, 1849^[1] - Ciempozuelos, 1919)^[2] was a Spanish architect,^[3] born to jurist José Benito Lázaro and astorgana María de Diego Pinillos.^[1] He was a disciple of Juan de Madrazo, a gothic revivalist in charge of León Cathedral's restoration,^[3][4] a project in which Lázaro de Diego took part, specializing in stained glass workmanship.^[3] He received a gold medal in the National Exhibition of Fine Arts of 1897 and the Great Cross of Isabella the Catholic in 1901.^[5]

From 1875 to 1879 he worked as town-architect in Ávila.^[6] He developed a remarkable activity in the restoration of architectural heritage in the late nineteenth century.^[7] One of his biggest contributions was the introduction of Catalan vault system in Madrid,^[7] after travelling to Barcelona in 1888.^[5] He had a stained glass workshop in Ayala street, Madrid, run from 1890 to 1911.^[3]

Some of his works are:

• the School of the Ursulinas,^[3][8]
• the Chapel of Saint Diego and Saint Nicholas asylum,^[3][9]
• Church of La Milagrosa,^[10]
• Church of El Pilar,^[11]
• Church of Los Redentoristas,^[11]
• Church of Las Reparadoras,^[12]
• Church of Las Hijas de la Caridad,^[13]
• Church of Cedillo,^[14] Temple of Sacred Hearts in Sabucedo de Montes,^[15] the enlargement of Ávila's cemetery^[16] or La Bañeza's cemetery.^[17]

In his last years he developed a mental illness,^[18] that led him to be interned in the psychiatric sanatorium of San José de Ciempozuelos in 1908, confinement that would last until his death in 1919.^[19]