The chancellor candidate earned a total of €1.25 million from 89 speaking engagements since he left office as Merkel's finance minister in 2009, the Bild newspaper revealed ahead of Tuesday's announcement.
He earned an average of €14,065 per speech, the paper said, with fees for individual speeches ranging from €1,000 to €25,000. Adjusted for a 48.5 percent tax rate, that left him with an average of €7,314 per speech.
In some cases, Steinbrück also donated the fee to social projects, including the Catholic charity "Tafel Monheim", the family counselling service "Pro Familia", and a jazz museum.
Steinbrück also gave 237 unpaid speeches during that period, at schools, the SPD's political foundation Friedrich-Ebert-Stiftung, and in some charities.
These fees come on top of his salary as an MP in the Bundestag which amounts to €7,960 a month plus expenses. As a former state secretary, former minister in two different states, former state premier of North Rhine-Westphalia, and former federal finance minister, he is also entitled to a number of different pensions, amounting to around €1,800 a month, pension experts estimate.
The disclosure of his exact income comes after a row erupted in parliament over his earnings, and a debate about how much MPs should be allowed to earn on the side.
The government coalition recently announced a reform in the bracketing structure by which MPs' incomes are revealed, but fell short of imposing full disclosure.