His father, Albert II, abdicated the throne over concerns about his age and health, CNN reported.
"I have noticed how my age and my health have not permitted me to exercise my duties the way I would like to," Albert, 79, said in an address to the nation.
European Commission President Jose Manuel Barroso issued a statement praising Albert's decision to step down.
"I wish to express to him my sincere thanks for his tireless efforts in the service of Belgium and Europe," the statement said. Albert "has never ceased to be a staunch defender of the values that are the basis of the European construction."
Along with a new king, Belgium also gained a queen Sunday in Philippe's wife, Mathilde.
Phillipe, 53, attended public schools in Belgium's two major languages, Flemish and French, and later studied at the royal military academy, where he trained to be a fighter pilot.