The Financial Tim
es in London first reported the move on its website, saying it would be the first of a series of positions Blair expects to take in the private sector.
Blair's income from the job was not disclosed, but the FT reported that one New York recruitment consultant said it was likely to be more than $1m a year.
The Financial Times quoted Jamie Dimon, JP Morgan's chief executive, as saying that Blair would be "enormously valuable" to the company.
"There are only a handful of people in the world who have the knowledge and relationships that he has," he said.
Blair's move comes one month after Jonathan Powell, his former chief of staff, joined Morgan Stanley, JP Morgan's rival, as a managing director in Morgan Stanley's investment banking division.
Blair, a key ally of George Bush, the US president, was replaced last year by Gordon Brown as prime minister after over 10 years as prime minister.