Lawmakers in Berlin's glass-domed Reichstag will vote from 0800 GMT to re-elect Merkel before she is appointed at 1000 GMT by President Frank-Walter Steinmeier and takes the oath of office an hour later.
For the veteran leader, the ceremony will mark the end of a painful stretch of post-election paralysis, the deepest crisis in her 12-year career.
A right-wing populist rise in a September election weakened all mainstream parties and deprived Merkel of a majority, forcing her into another unhappy alliance with the centre-left Social Democratic Party (SPD).
The grand coalition, mockingly dubbed a "GroKo", didn't start as a "love marriage", her designated vice chancellor and finance minister, the SPD's Olaf Scholz, drily observed Monday.
All coalition partners nonetheless sought to allay fears that their marriage of convenience could break up mid-term, insisting they plan to jointly govern until 2021.
"We have got a lot of work ahead of us," said Merkel.
And Horst Seehofer of her conservative Bavarian CSU allies declared it was time to "get a move on" after months of paralysis.
From 1600 GMT, Merkel sits down with her new cabinet, in which the SPD has wrested both the trophy posts of finance and foreign affairs to the dismay of a growing band of critics within her Christian Democratic Union (CDU).
In coming days Merkel will head to Paris to discuss EU reform plans with French President Emmanuel Macron ahead of a March 22-23 summit, after a six-months stretch in which Berlin was hamstrung on the European and world stage.