The IOM said it had recorded 432,761 arrivals in Europe since January 1, comprising 309,356 in Greece, 121,139 in Italy, 2,166 in Spain and 100 on the island of Malta.
Last week, IOM had recorded 364,000 arrivals, but spokesman Daniel Szabo told AFP the numbers were revised upwards because the Greek government had significantly improved its counting methods in recent days.
The rising figures were not caused a sudden surge of migrants, he said.
On Thursday, Greek officials said some 22,500 had been registered on Lesbos already this week.