Francois Bayrou, who was education minister when Sarkozy was communications minister from 1993 to 1995, said he was voting for the Socialist candidate because of Sarkozy's rightward shift after the first round of elections, Radio France Internationale reported Friday.
Bayrou, who polled fifth in the first round April 22, said his decision was personal and he did not urge his supporters to vote for Hollande.
"I, personally, will vote for Francois Hollande," he said. "After a good result in the first round, Nicolas Sarkozy set off in chase of an extreme right within which we do not recognize our values and in which our deepest and most precious beliefs are battered and denied."
Sarkozy called Bayrou's decision "illogical," RFI said.
Recent polls indicate Hollande leads Sarkozy going into the runoff.