Celta will pay €12.9 million plus €3.1m in add-ons for the 25-year-old.
Denis struggled for minutes last season and joined Arsenal on loan in January but had a pretty tough time with the Gunners.
He will now be hoping he can enjoy some regular football at Balaidos to get his career back on track.
Denis joins Marc Cucurella, Marc Cardona, Andre Gomes and Jasper Cillessen in departing the Camp Nou this summer.
Good luck at Celta Denis Suarez!