Martin Bashir, a Christian, rose to prominence in 1995 when he interviewed Diana, Princess of Wales for the corporation's flagship investigative programme Panorama.
He said he was "delighted" to be returning to the BBC, for which he was a news correspondent from 1987 until 1992.
He also presented BBC Radio 4's Sunday programme until 2004.
In his new role, Mr Bashir will report on events and provide analysis and insight on the major themes and issues affecting different faiths in the UK and around the world.
He said: "The opportunity to cover the broad spectrum of religious affairs is challenging and compelling and I cannot wait to get started."
The BBC's head of newsgathering Jonathan Munro said: "Martin's track record in enterprising journalism is well known and respected in the industry and amongst our audiences.
"As a student of theology, Martin will bring immense knowledge of the brief to his new role, and an enthusiasm to cover the broadest range of faith-based stories."
Mr Bashir's interviewees have also included the suspects in the murder of Stephen Lawrence and singer Michael Jackson.
The journalist was a correspondent for Tonight on ITV before moving to America and hosting ABC's Nightline programme in 2004.
In 2010, he joined NBC News as an MSNBC anchor and a correspondent on the Dateline programme.