Several hundred thousand people were expected in the northern city of Bremen, which is hosting the celebrations this year, for street parties and events led by Chancellor Angela Merkel, who was brought up in East Germany.
In a video podcast on the eve of the celebrations, Merkel paid tribute to East Germans for having "the courage to fight for freedom."
"At the same time, there was a huge wave of solidarity from the people in West Germany," she said. "It is thanks to these joint efforts that we have been able to rebuild so quickly and make Germany a country that is respected in the world."
Also expected to attend the reunification celebrations were European Commission President Jose Manuel Barroso and NATO Secretary General Anders Fogh Rasmussen.
US President Barack Obama congratulated the country, praising "the courage and conviction of the German people that brought down the Berlin Wall, ending decades of painful and artificial separation."
The peaceful reunification of East and West Germany was a "historic achievement," he said.
The reunification treaty was signed on October 3, 1990, just under a year after the Berlin Wall was pulled down in a bloodless revolution, and brought the two halves of the country together amid joyful scenes.
In a recent poll, 84 percent of Germans said they believed national unification after four decades of division had been the right decision, despite a lingering economic gap between East and West.
Just 14 percent said unity had been a mistake, according to the survey for ZDF public television.
There were, however, protests against the reunification festivities, with around 1,800 mainly left-wing activists conducting a march that passed off peacefully.