Fortis is selling its French operation to Swiss Life in a deal worth up to E200m (£122.7m).
The exact price, between E150m (£92m) and E200m, will be settled after the company's accounts for 2001 have been closed.
Fortis France's main asset was a life insurance company, Fortis Assurances.
A group spokeswoman said the deal was in line with Fortis' strategy to concentrate on core markets.
There were no immediate plans to sell off UK operations, but the company was keeping its options under review, she said.
Fortis France had 0.6% of the French life insurance market.
She said: "We want to be leaders in our core markets. Given the characteristics of the French market, it would have been quite difficult to grow to a leading position, so we decided to look at other possibilities.
"We don't have any similar plans in the UK right now but Fortis is reviewing all its operational units to see if they fit in with its strategy."
The strategy for the Benelux-based group was outlined in December 2000 by chief executive Anton van Rossum, who joined the group in September 2000.
The deal with Swiss Life does not affect Fortis's other French operations, which include Fortis Banque, Fortis Securities, Fortis Investment Management, Fortis Lease, Fortis Commercial Finance and Mees Pierson.