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.
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.
He stressed his wish that Belgium – split between the Dutch-speaking north and the French south – remained united.
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