The Codex Sinaiticus, written in Greek on parchment leaves in the fourth century, is available online in a project involving institutions in Britain, Germany, Egypt and Russia which held different parts of the ancient book.
As part of the four-year joint project, digital photographs have been taken of the reunited manuscript, allowing scholars worldwide to research in-depth the Greek text, the British Library in London said.
The library, which holds a large chunk of the Bible, also opened an exhibit Monday that includes artefacts linked to the manuscript to coincide with its online launch.
"The Codex Sinaiticus is one of the world's greatest written treasures," said Scot McKendrick, head of Western manuscripts at the British Library.
"This 1600-year-old manuscript offers a window into the development of early Christianity and first-hand evidence of how the text of the Bible was transmitted from generation to generation," he said.
Originally 1,460 pages long and measuring 16 inches (40 centimetres) by 14 inches, the manuscript was handwritten by a number of scribes around the time of Constantine the Great who died in 337, experts said.
The manuscript, which was revised and corrected over the centuries, lay undisturbed in a monastery in Sinai in Egypt until it was found by a German professor in the mid-1800s and handed to Russia's Tsar Alexander II.
Britain later bought most of the book from the Soviet Union in the 1930s, while Egypt kept still more pages found in the monastery in 1975.
Professor David Parker, whose team made the electronic transcription of the manuscript, said the Internet project proved challenging with some of the pages in poor condition.
"The process of deciphering and transcribing the fragile pages of an ancient text containing over 650,000 words is a huge challenge, which has taken nearly four years," said Parker from the University of Birmingham.
"The digital images of the virtual manuscript show the beauty of the original and readers are even able to see the difference in handwriting between the different scribes who copied the text," he said.
The manuscript is available at http://www.codexsinaiticus.org.