The computer software tycoon was splatted as he got out of his car to attend a reception in Brussels on Wednesday.
Police arrested one pie thrower and a woman who was photographing the incident.
Responsibility for the attack was quickly claimed by Belgian practical joker Noel Gaudin, who has previously organised a series of creamings of celebrities he considers take themselves too seriously.
Mr Gates was his most famous victim to date. Past targets have included the French philosopher Bernard-Henri Levy and French news presenter Patrick Poivre d'Arvor.
Mr Gates, who was in the country for talks with the Prime Minister, Flemish government officials and businessmen, is said to have been left speechless by the attack.
"After taking four pies straight in the face he could hardly say anything," Mr Gaudin said.
However, after a quick clean-up Mr Gates was back on track and re-appeared at the function hosted by the Flemish community.
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