Seen as a prickly but bright politician who once served under Merkel as Germany's finance minister, Steinbrück won plaudits for his handling of the 2008 financial crisis. But the 65-year-old father-of-three from the northern German city of Hamburg now has the uphill task of stopping the world's most powerful woman from winning a third term.
He was nominated officially by his centre-left Social Democratic Party (SPD) at a special conference on Sunday in Hannover, with 93.45 percent of the party faithful behind him. Burly and bespectacled, Steinbrück has an abrasive manner that should stand him in good stead when battling the German chancellor on the hustings and spice up the campaign for the election expected to be held in September.
Voters recently had a taste of his fiery debating style in the Bundestag lower house of parliament, notably when he accused Merkel of doing the "dance of the seven veils" on saving Greece and challenging her to "tell it how it is." He has also vowed to make the occasionally drab world of German politics somewhat more colourful, saying: "This should not be a boring election campaign. This should not be a humourless campaign. There must be fun and a few jokes."
And he showed plenty of touches of his acerbic sense of humour during a speech lasting nearly two hours ahead of his nomination.
"Mrs Merkel said she is running the best and most successful government since reunification (in 1990). I have rarely laughed so much," said Steinbrück to tumultuous applause. But he has an uphill task ahead of him, with opinion polls suggesting Merkel's conservatives enjoy a significant lead over the SPD, which has found it difficult to dent her popularity.
Off to a rocky start
He has not been helped by a rocky start to his campaign, first accused of using his influence when finance minister to gain sponsorship for a chess match, then admitting he earned €1.25 million ($1.61 million) for speeches.
"In the eyes of many citizens, I'm a wealthy man," Steinbrück said in his speech in Hannover. "That's correct, but what does a bank statement, what does a standard of living say about a person's readiness and ability to help those citizens who are worse off? Nothing. Nothing at all."
Seen as being towards the right of the centre-left Social Democrats, Steinbrück will have to work hard even to persuade his own party faithful to back him, said political analyst Gerd Langguth.
"He will try to pull together the different wings of the party, especially the left wing. His problem for the campaign is that he doesn't have the whole party behind him," he said.
Seeking to appeal to the left, Steinbrück said he would put social justice and equality at the heart of his election bid.
During his stint in Merkel's coalition cabinet, the trained economist worked closely with her to mitigate the effects of the financial crisis caused by the 2008 collapse of US investment bank Lehman Brothers, and enjoyed a high international profile. Many in financial circles considered them the "dream team" to lead Germany out of the crisis.
It was "his biggest political act", said one of his biographers, Daniel Friedrich Sturm. He acted with "composure, assurance and determination" to put out the fires, said the author. Like Merkel, "he analyses situations soberly, understands everything very quickly and has a slightly cynical sense of humour," he added.
A man of action
Gero Neugebauer, political scientist at the Free University in Berlin, also said Steinbrück's style was similar to that of Merkel's, although more dynamic. "If Merkel is often described as hesitant, Steinbrück is a man of action," said Neugebauer.
But his sense of humour has at times landed him in hot water and led to one newspaper dubbing him the "most hated man in Switzerland" after he compared the Swiss to "Indians" during a bitter tax row between the two neighbours.
And his off-the-cuff comments to journalists have also prompted unintended consequences. During a period of volatility on the foreign exchange markets, Steinbrück joked: "I love cash and I love a strong euro," causing pandemonium on trading floors worldwide, as twitchy analysts interpreted the joke as German acceptance of a high exchange rate.
A sociable man, who enjoys the occasional cigar and whose briefings to journalists over a few beers have been known to extend late into the night, he was said to have good relations with Merkel when they worked together. He quipped that he hopes to have a glass of wine with her after he wins the election, to toast her years as chancellor.
SPD supporters will hope that his campaign follows the example of his favourite animal, the rhinoceros.
"It starts very slowly but when it gets going, nothing can stop it," said Steinbrück.