The 26 year-old Argentina moves to Spartak in an outright transfer.
Pareja said: "It is a step forward in my career, because I am going to a league that is growing and to a top club playing in the Champions League next season. It is a challenge.
"I feel great sadness for all that I leave here: family, friends and a team that I doubt is like any other.
"All employees of the club have treated me wonderfully. I spent two incredible years here."