The Constitutional Council also rejected calls for a referendum by political opponents but struck out some of the reforms citing legal flaws.
Twelve days of protests have been held against the reforms since January.
In March, the government used a special constitutional power to force through the changes without a vote.
President Emmanuel Macron argues the reforms are essential to prevent the pension system collapsing and Prime Minister Élisabeth Borne tweeted on Friday that "tonight there is no winner, no loser".
