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.
