A day after Guttenberg quit in the wake of accusations he plagiarized chunks of his doctoral dissertation, Merkel decided to hand her trusted lieutenant de Maizière, 57, the key defence portfolio.
"I don't just appreciate Thomas de Maizière's brilliant intellect and exemplary sense of duty and responsibility," Merkel said on Wednesday afternoon. "But he's also known for pursuing policies based on a foundation of solid values."
Merkel said she was certain de Maizière would quickly win the trust of the nation's soldiers while pushing forward crucial military reforms.
Replacing him as interior minister will be Hans-Peter Friedrich, 53, parliamentary group leader of the Christian Social Union (CSU), the Bavarian sister party to Merkel's conservative Christian Democrats (CDU).
Since Guttenberg belongs to the CSU, appointing CDU man de Maizière to the Defence Ministry entailed handing the Interior Ministry to a Bavarian conservative in order to preserve the balance of Merkel's three-party, centre-right coalition.
De Maizière served as Merkel's chief of staff at the Chancellery during her first term, but he switched to the Interior Ministry after her re-election in September 2009.
Guttenberg, 39, dramatically resigned on Tuesday, depriving Merkel's government of its most popular figure with six state elections looming this year. He also leaves the defence portfolio in a state of disarray, with the German military embarking on the biggest reform programme in its history.
Besides scrapping conscription, Guttenberg proposed radically downsizing the nation's armed forces to make the Bundeswehr a more effective professional force.
He also wanted to increase the number of troops available for foreign missions so as to make the Germany military better suited to 21st century security challenges.
His plans, part of an €80-billion package of government cuts, had run into trouble however, with reports this week that Chancellery officials had sharply criticised them as rudimentary.