Juan Bautista Lázaro de Diego (León, 1849 - Ciempozuelos, 1919) was a Spanish architect, born to jurist José Benito Lázaro and "astorgana" María de Diego Pinillos. He was a disciple of Juan de Madrazo, a gothic revivalist in charge of León Cathedral's restoration, a project in which Lázaro de Diego took part, specializing in stained glass workmanship. He received a gold medal in the National Exhibition of Fine Arts of 1897 and the Great Cross of Isabella the Catholic in 1901.
From 1875 to 1879 he worked as town-architect in Ávila. 
He developed a remarkable activity in the restoration of architectural heritage in the late nineteenth century. One of his biggest contributions was the introduction of Catalan vault system in Madrid, after travelling to Barcelona in 1888. He had a stained glass workshop in Ayala street, Madrid, run from 1890 to 1911. 
Some of his works are:
In his last years he developed a mental illness, 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.
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