His wife Cherie is a Catholic and there had been speculation that he would convert to Catholicism from his Anglican faith after leaving office.
Mr Blair's official spokesman confirmed he had converted on Friday night and said it was a private matter.
Mr Blair, who is now a Middle East peace envoy, met Pope Benedict XVI in June. Cardinal Cormac Murphy O'Connor is expected to issue a statement later.
Earlier this year, the former prime minister told the BBC that he had avoided talking about his religious views while in office for fear of being labelled "a nutter".
His ex-spokesman Alastair Campbell once told reporters "We don't do God," but has since said that his former boss "does do God in quite a big way".
Güncelleme Tarihi: 22 Aralık 2007, 22:19