Darden, 31, was in his first season with Zalgiris and was second on the team in scoring and rebounds through 20 Euroleague games.
The club said that Darden had exercised his right to unilaterally terminate his contract upon receiving an offer from another club.
Darden was averaging 9.9 points and 3.9 rebounds while also leading Zalgiris in two-point (66.7%) and three-point accuracy (51.7%). He ranked fourth among all Euroleaguep players in true shooting percentage with 9.9 points against 15.7 attempted per game.
Darden had made his Euroleague debut with Unicaja Malaga last season after seven previous seasons as a pro in Turkey, Belgium, Australia and France.
The player is expected to join Real Madrid, a team against whom he had his best game this season in Euroleague finishing with 20 points, 10 points and an index rating of 24. Darden will be eligible for Liga Endesa games only.