On his Twitter account, the king wished the Belgian team success in the tournament, adding that he would see them during Saturday's game.
Belgium will face Tunisia at Moscow's Spartak Stadium on Saturday.
Belgium beat Panama 3-0 while Tunisia lost 2-1 to England in the teams' first matches at the tournament.
Russia is hosting its first FIFA World Cup, which kicked off on June 14 and will continue until July 15 in 11 cities across the country.