Quassaick Creek is an 18.4-mile-long (29.6 km) tributary of the Hudson River in Orange and Ulster counties in the U.S. state of New York. It rises in the glacial ridges west of the river, near the boundary between the towns of Plattekill and Marlborough. From there it flows south into the town of Newburgh and then the city, where it eventually forms part of the border between it and neighboring New Windsor before emptying into the Hudson.