Olaf Scholz, 59, the interim leader of the Social Democrats (SPD), will take over the powerful finance ministry which in recent years was led by fiscal hawk Wolfgang Schaeuble of Merkel's Christian Democrats (CDU).
Heiko Maas, the SPD's outgoing justice minister, will take the prestigious post of running the foreign ministry, according to the party's designated next leader Andrea Nahles.
Both Merkel's conservative CDU/CSU bloc and the SPD, the two mainstream parties in Europe's biggest economy, lost millions of votes to far-right populists in last September's election.
Merkel, aged 63 and in power for 12 years, is due to formally launch her fourth-term government next Wednesday.
But unrest is brewing within her party following a historically poor election result and Merkel's decision to give up key cabinet posts as the price of holding onto power.
Given the mutual distrust between the two big parties, "it's possible that this coalition won't last four years," one senior CDU source has told AFP. "Unlike in the past, I'm not sure this time."
Scholz said the SPD's ministers, three men and three women, were "a good team of people with a high level of expertise and the ability to run large organisations".
Nahles then formally announced that Scholz, the mayor of the city-state of Hamburg, would take over the finance ministry, which has been led in recent months by Merkel's deputy Peter Altmaier, 59.
She said Scholz had led the wealthy river port "with a keen eye on finances, with budgetary discipline on the one hand and future-oriented investments in education, research and infrastructure on the other".
She said this would help Maas, who has little international experience, in a job where Germany increasingly acted "as a mediator in a disorderly world".
Taking over the justice portfolio will be Katarina Barley, 49, a former judge, while Hubertus Heil, 45, becomes labour minister and Berlin local politician Svenja Schulze, 49, takes on the environment post at a time Germany is on course to miss its ambitious climate targets.
Nahles -- soon to become the 153-year-old labour party's first female leader -- has shown, by sending her nemesis Gabriel to the backbenches, that she knows "revenge is a dish best served cold", judged Spiegel Online.
Among Merkel's conservatives, Ursula von der Leyen, 59, stays on in the defence ministry and the chancellor's loyal right-hand man Altmaier is rewarded with the economy and energy ministry.
Andreas Scheuer, 43, of Merkel's conservative Bavarian sister party the CSU, will become transport minister, wresting with Germany's auto pollution scandal, while Gerd Mueller, 62, stays on as development minister.
Crucially, the embattled outgoing CSU chief Horst Seehofer, 68, as Germany's next interior minister in charge of security takes over a rebranded "homeland" ministry in a bid to answer the challenge of the new far-right.
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