Founding Under Sixtus IV

Pope Nicholas V, who reigned from 1447 to 1455, envisioned Rome as a renewed center of humanist scholarship and began assembling a substantial personal and papal manuscript collection intended to serve as a public library for the city, but his death left the project incomplete. His eventual successor Pope Sixtus IV, who reigned from 1471 to 1484, took up and formally realized this vision, issuing the bull Ad decorem militantis ecclesiae on 15 June 1475 to establish the Vatican Library as a permanent institution and appointing the humanist scholar and historian Bartolomeo Platina as its first librarian.

Platina organized the growing collection into four rooms according to subject and access: separate libraries for Latin and Greek works available to readers, a Bibliotheca Secreta for particularly precious manuscripts not available for general consultation, and a Bibliotheca Pontificia holding the papacy's own administrative archives and registers. The library grew rapidly under this organized structure, from roughly 2,500 manuscripts recorded in 1475 to around 3,500 by 1481 according to Platina's own inventory, already making it one of the largest manuscript collections in Western Europe within a few years of its formal founding, a growth that continued through subsequent centuries as the papacy acquired entire private libraries and specialized collections.