Air France currently allows obese people to buy two seats at a discount
Air France is to offer a free second seat to obese passengers "for their own comfort", the airline has said.
Overweight passengers can already buy an extra one with a 25% discount, but from 1 February, this will be refunded if their flight is not full.
It also confirmed that, in common with other airlines, an obese traveller could be prevented from flying.
This was to ensure a flight can be evacuated within 90 seconds, the BBC's transport correspondent said.
It came down to the discretion of the captain whether to let them fly or tell them to disembark for safety reasons, Richard Scott said.
The airline said it had no intention of forcing heavier passengers to pay for a second seat.
It explained that if an overweight person turned up without having booked an extra one, the check-in staff would not intervene.
The passenger would continue to the plane, where the airline said "in 99% of cases" on-board staff would work something out as planes were rarely completely full so they would be able to position them next to an empty seat.
Other non-overweight passengers could also book a second seat for a 25% discount, Air France said.
But they would not get the refund if the plane was not full when it took off.
Air France is part of Air France-KLM, the largest airline group in Europe by revenue.