After a ceremonial handing over of the keys, lawmakers held their inaugural session in the landmark building, which has gone through a $330 million renovation that includes a sparkling new glass dome on top of the original 19th-century, neo-Renaissance facade.
Chancellor Gerhard Schroeder spoke to lawmakers about German unity - 10 years after the fall of the Berlin Wall.
He stressed, however, that the migration of government from Bonn to the former capital of Kaisers and of Nazi and Communist dictators would not disrupt the democratic tradition established in western Germany over the past half-century.
While the return to the old capital has been heralded as a milestone in Germany's postwar development into a strong democracy, celebrations over the launching of the so-called "Berlin Republic" have been muted by the NATO bombing of Yugoslavia.
As part of that campaign, German forces are engaged in their first combat mission since World War II.
"I really would have hoped for more peaceful times for this pleasant occasion," parliamentary President Wolfgang Thierse said on German television Sunday night. "But one can't pick and choose and one also can't look away and want to flee."
The reopening of the Reichstag is the first big step in the government's move from Bonn, the provincial western city where West Germany's capital was relegated after World War II.
But lawmakers are staying only for a day, and the building itself will be open to the public for only a week before closing again for final touches.
Lawmakers will meet there again in May to elect Germany's new president, but won't hold regular meetings there until the new session convenes in September.
Still, Monday's ceremonies are the biggest thing to happen to the Reichstag since the eccentric artist Christo wrapped it in a million square feet of silvery fabric in 1995 to celebrate the end of communism.
The Reichstag was built in 1894 to house the first German parliament, an attempt to install elements of democracy under Kaiser Wilhelm. Real democracy took hold - however briefly - after World War I, when the empire was abolished and the Weimar Republic set up in 1919.
Adolf Hitler used a 1933 fire that gutted the building to suspend civil liberties and consolidate Nazi rule. During World War II, it was bombed by the Allies and, at the end, stormed by Red Army soldiers, who scrawled obscenities on the walls.
Some of that vandalism has been preserved in British architect Sir Norman Foster's renovations.
The architectural mix of old and new in large part embodies the professed spirit of Schroeder's center-left government. The frst German chancellor too young to have personal memories of World War II, Schroeder has stressed that enough time has passed for Germany to be powerful without being threatening, that it can remember its crimes without being defined by guilt.
The transformed Reichstag is as light as its legacy is dark. Glass walls around the central parliament. A viewing platform on the second floor means visitors can see and hear what's going on inside.
The feel is one of openness, of a transparent and accessible government.
By ANNE THOMPSON