France's defence ministry disputed the finding, saying the 3 January strike targeted an "armed terrorist group".
France has 5,100 troops across the Sahel region to fight militants tied to al-Qaeda and the Islamic State group.
The UN carried out an investigation after locals alleged that a helicopter had fired on people attending a wedding near Bounti village in central Mali.
The investigation found that a wedding had taken place, and there had been "about 100 civilians at the site of the strike", AFP news agency quoted the UN report as saying.
About five armed people, thought to be members of a jihadist group affiliated to al-Qaeda, also attended the celebrations, the report said.
"The group affected by the strike was overwhelmingly composed of civilians who are protected persons under international humanitarian law," the UN said.
"This strike raises serious concerns about respect for the principles of the conduct of hostilities," it added.
In a statement, France's defence ministry said it had "numerous reservations about the methodology used" by UN investigators.
It "maintains with consistency and reaffirms strongly" that the air strike targeted an "armed terrorist group", the ministry added.
The UN investigation was carried out by the human rights division of its mission in Mali, Reuters news agency reports.
Its team analysed satellite images, visited the area in late January and interviewed more than 400 people.