All 108 delegates voted, with 78 approving the plan and 30 opposing.
The committee backed the view that marriage is a union between a man and a woman.
Although a vote had been planned for Thursday, so many representatives asked to address the Synod that the debate was scheduled to continue late into the evening and the vote delayed until Friday.
Roughly eight out of 10 people in Finland are registered as Lutheran. The church suffered an upswing in resignations this autumn linked to gay rights issues.