"Everyone knows how much Ukraine means to me. What I've rarely talked about is what the war has done to me on the inside," Svitolina writes in the post.
Svitolina, 32, a singles player born in Odesa and ranked sixth in the world, has often spoken out against Russia's war of aggression and has refused to shake hands with Russian opponents.
In the post, she writes about doubts and sleepless nights, and says Monfils has always been by her side.
"Over time, I realized that I had to find myself again and that I couldn't do it within our marriage," Svitolina writes.
Monfils, 40, was coached by Swede Mikael Tillström and recently announced that he was ending his professional career. His highest ranking was sixth.