Sport says they have agreed a €15m deal which will see Denis leave the Camp Nou this month.
Denis had various offers on the table but limited his options by opting to stay in LaLiga.
After failing to agree personal terms with Valencia, Celta moved ahead in the pecking order and quickly found common ground with both Barça and the midfielder.
Barça were keen to get a transfer completed before 1st July in order to balance out the accounts for the 2018/19 season. The deal will go through on Sunday 30th June.