Much of the initial reaction came from the Danish capital, Copenhagen, where European Union leaders were meeting.
"I have sent my congratulations,", European Commission President Romano Prodi said.
President Jacques Chirac of France, whose chances of a centre-right ally in Germany were dashed, dispatched a hand-written letter of congratulation to Mr Schroeder, calling his election win "welcome".
"In this decisive period for the European Union - which is on the eve of a major expansion and a profound reform of its institutions - our two countries must continue to play the role of the driving force behind the construction of Europe," the letter read.
A spokesman for the UK Government said Prime Minister Tony Blair welcomed the result and looked forward to continuing his "very good working relationship" with Chancellor Schroeder.
Sweden's Prime Minister Goeran Persson, himself re-elected last week for a centre-left party, was effusive.
"I am extremely happy to have one more new Social Democratic election victory in Europe," Mr Persson said.
Ireland's Bertie Ahern did not seem at all concerned that Mr Schroeder's much-reduced majority of just nine seats could reduce the effectiveness of his government.
"In Irish terms that would be a good result," the Irish premier said.
In Moscow, Russia's President Vladimir Putin telephoned Mr Schroeder to offer his congratulations.
The two leaders expressed a commitment to further, gradual development of the relationship between their countries, the Russian news agency Interfax reported.
Among the countries wishing to join the European Union, Poland - which is keen for German support in its membership bid - was the first to react
The German people had made a "good choice", said Prime Minister Leszek Miller.
"Personal contact, sympathy - these are... important things in politics," Mr Miller said on Polish radio.
"From this point of view we are friends with Gerhard Schroeder and [Foreign Minister] Joschka Fischer. There is between us a link of sympathy and understanding."
'A lot of work'
But the United States was critical of Mr Schroeder's campaign strategy of pledging opposition to a US-led attack on Iraq.
"At the governmental level, Chairman Schroeder and his government have a lot of work to repair the damage that he did by his excesses during the campaign," White House spokesman Ari Fleischer said.
Defence Secretary Donald Rumsfeld, meanwhile, described the way the campaign was conducted as "unhelpful" and said that it had had the effect of "poisoning the relationship" between the two countries.
Mr Rumsfeld also delivered what appeared to be a snub to the new Berlin administration, by saying he had no plans for a meeting with German Defence Minister Peter Struck.
Another note of caution was struck by Italy's European Affairs Minister, Rocco Buttiglione, in a newspaper interview published on Monday.
Mr Schroeder's stance on Iraq would have a "bad effect" on the EU's security policy, he told the Corriere della Sera newspaper.
"We will have to split on this point because it is important that there are no divisions between the United States, the United Nations and Europe over Iraq," he said.
China warmly congratulated Mr Schroeder's Social Democrat party and its coalition partners, the Greens.
"The Chinese Government will make efforts with the German Government led by Chancellor Schroeder to promote the development of friendly relations between the two countries to mark the 30th anniversary of the setting up of diplomatic ties between China and Germany," a Foreign Ministry statement said.