It is understood that De Boeck, who had been in the position since December 2008, is being replaced because his background is in banking, an area Fortis is no longer involved in as a result of the credit crunch.
Bart De Smet, who currently leads the Belgian insurance operation, will take over as CEO on July 1.
De Smet will become the fifth CEO of the firm in the past 12 months.
Chairman Jozef De Mey says: “By appointing Bart De Smet, a seasoned executive who has spent his entire career within the insurance industry, we ensure that we have the right management in place to develop our position as an international insurance group.
“Karel played a critical role in steering Fortis through an extremely difficult period. He took over as CEO when the future of the company was most uncertain and he successfully negotiated the restructuring of the Group and guided it through a complex shareholders’ approval process. The board is grateful for the great work he has done, leaving to his successor a strong insurance group with encouraging prospects for future growth.”