The Oxford- and Stanford-educated, trained air force pilot took the oath as the country's seventh king in a ceremony in parliament.
To warm applause, King Philippe, 53, promised to uphold the constitution.
Belgium has a constitutional monarchy in which the king plays a largely ceremonial role.
One of the duties the monarch does have is trying to resolve constitutional crises.
In his final address before signing a legislative act to step down, 79-year-old King Albert said his country must remain a "source of inspiration" to Europe.
He stressed his wish that Belgium - split between the Dutch-speaking north and the French south - remained united.
His resignation on the grounds of ill-health came after nearly 20 years on the throne and was timed to coincide with Belgium's national day. (+ video) » | Sunday, July 21, 2013
THE SUNDAY TELEGRAPH: Divided Belgium has a new King Philippe: Belgium has a new King Philippe of the Belgians after his father Albert II abdicated after making an emotional exhortation for his countrymen to "work tirelessly" to hold their divided country together. » | Bruno Waterfield, Brussels | Sunday, July 21, 2013
ARD: TAGESSCHAU: Thronwechsel in Belgien: Philippe ist König » | Sonntag. 21. Juli 2013
LES ECHOS.fr: La Belgique célèbre son nouveau roi Philippe: Albert II a abdiqué dimanche en faveur de son fils Philippe, qui a prêté serment comme nouveau roi des Belges dans un pays divisé. » | dimanche 21 juillet 2013
Crying Queen of Belgium breaks down as she watches her husband abdicate and hand over the throne to their son » | Stuart Woledge | Sunday, July 21, 2013
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