Arnold is striving to feature for the Wallabies for the Rugby World Cup next year, but a switch to France will make the towering lock unavailable for international selection after the tournament in Japan.
The 28 year-old, capped 19 times by the Wallabies, said it was a tough call to leave his homeland.
"I've come to the decision that the 2019 season will be my last in Canberra. It was a very hard decision to make." Arnold said.
"The Brumbies have been great to me on and off the field and I'll always be grateful for that. I believe the timing is right to move on after this season and experience something new.
"Now that I've made my decision, I'm looking forward to the 2019 season and the future."
Brumbies chief Phil Thomson said: "Rory has the made the decision that he feels is best for the longevity of his rugby career.
"We would like to extend our best wishes to Rory and thank him for his commitment to the Brumbies for the past four seasons. We are looking forward to him performing strongly for us during the 2019 Super Rugby season."