The Dutchman was originally due to take over on January 1 after his contract with Zenit St Petersburg expired but he was sacked by the Russian club in August following a run of poor results.
Stand-in Belgium boss Frank Vercauteren resigned after a 2-1 defeat away to Armenia in a Group Five World Cup qualifier last week meant they could not qualify for the 2010 finals.
Advocaat will lead Belgium in their final two qualifiers at home to Turkey on October 10 and away to Estonia four days later.
Belgium's last appearance at a major championships was in the 2002 World Cup finals.
Former Netherlands and South Korea coach Advocaat had taken over as coach at Zenit in 2006, leading them to the Russian title in 2007 and the UEFA Cup the following year.
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