Ellenson played 17 games for New York this year after signing with the team in February. He joined the Knicks on a 10-day contract in February, then inked a multi-year deal in early March. He averaged 6.0 points and 3.4 rebounds in nearly 14 minutes per night.
Ellenson, 22, was taken by the Pistons with the 18th pick in 2016. He played 59 total games for Detroit, including two this season, before being waived in February.
In recent days, the Knicks have also declined options on John Jenkins and Billy Garrett as they try to maximize cap space heading into free agency. New York also opted not to extend qualifying offers to Emmanuel Mudiay and Luke Kornet.