The dramatic close-up of the former Labour leader, which measures 4ft by 3ft, is by artist Alastair Adams, who started work on it two years ago and had several sittings at Mr Blair’s home in Buckinghamshire.
After establishing the definitive pose, Adams used photographs to come up with the completed oil painting.
The gallery said the result was a “very immediate portrayal of the longest-serving Labour prime minister and, to date, the youngest Labour prime minister to take up office since 1812”.
It is the gallery’s first painted portrait of Mr Blair, who had been unwilling to sit for a portrait when he first left office.
Sarah Howgate, its contemporary curator, said the portrait reflected Mr Blair’s personality. “The direct gaze of the sitter is uncompromising but also reflects his considerable skill as a negotiator on the world stage,” she said.
“The gallery is now able to represent Tony Blair with a portrait consonant with the personality of an individual who has considerably shaped the political, economic and cultural climate of Britain.”
The gallery already has several photographic portraits of Mr Blair, including by Nick Danziger, Terry O’Neill and Eamonn McCabe.
First elected as an MP in 1983, Mr Blair led his party to three general election victories and was prime minister for ten years.
The portrait is in keeping with the gallery’s aim to have painted portraits of all former prime ministers. As yet, it does not have one of Gordon Brown.