Merkel's rival in the September election - who is running for public office for the first time - is the affable but guarded protege of her charismatic Social Democratic predecessor Gerhard Schröder.
The former chancellor made Steinmeier one of his closest advisors during his seven
years in power, and eventually his chief of staff, a job in which he coordinated the security services and shaped the Agenda 2010 package of economic reforms - the unpopular centrepiece of the administration.
He emerged from the shadows after the inconclusive 2005 general election, when Schröder put him forward as his candidate to take over the foreign ministry in negotiations on the formation of a "grand coalition."
A jurist with a cautious streak, Steinmeier finds it difficult to speak in media-friendly soundbites, preferring measured statements with elliptical German sentences that end far from where they started.
He more resembles a civil servant than a glad-handing politician, and his charm works better in smaller, relaxed contexts where admirers say his dimpled smile and genuine interest in people can be used to the fullest.
"I never intended to become a politician," Steinmeier told glossy magazine Bunte in a photo spread showcasing the private side of the candidate. "These things just happen sometimes."
A born diplomat, Steinmeier has found it difficult to hone the killer instincts needed on the campaign trail. His Social Democrats are now trailing Merkel's conservatives by up to 15 points in opinion polls with less than two months to go until election day. Even supporters admit that taking on the high-flying Merkel may turn into a kamikaze mission on behalf of the party.
Born January 5, 1956, in a small town in Lower Saxony, Steinmeier was known on the football field as an efficient "all-rounder" who could play any position with ease and work well within a team.
The same qualities led Schröder to take him under his wing, first as media advisor when he was premier of Lower Saxony and later as a state secretary at the chancellery until becoming chief of staff in 1999.
He took over the foreign ministry in November 2005 and has had, time and again, to defend his remit against encroachment by Merkel. They clashed openly on Germany's approach to Russia and China, with Steinmeier warning against alienating either country with too strident criticism.
Steinmeier has also tried to bolster Germany's "soft power" with an emphasis on cultural diplomacy, frequently inviting painters, musicians and novelists to accompany him on official visits abroad and present the country in a more flattering light.
His profile grew again in 2007 as he became vice-chancellor when a party general, Franz Müntefering, stepped down to care for his ailing wife. Müntefering has since returned to become head of the party. But political scientist Peter Lösche said Steinmeier's latest incarnation as a candidate was doomed from the start.
"He is not a political campaigner," Lösche said. "He is very competent in many political areas but he is unable to project that to the outside world."
Married with a daughter, Steinmeier is protective of his private life although his wife Elke Büdenbender, a judge, has stepped up her public appearances since her husband announced he would stand for office.