The milestone marks another step in YouTube's evolution from a quirky start-up launched in 2005 to one of the most influential forces in today's media landscape.
Google bought YouTube for US$1.76 billion (S$2.22 billion) in 2006, when the video site had an estimated 50 million users worldwide.
YouTube crossed the 1 billion threshold five months after Facebook said its online social network had reached that figure. The vast audience has given YouTube's owner, Google, another lucrative channel for selling online ads beyond its dominant Internet search engine.