Union Bay is that part of Lake Washington west of a line drawn between Webster Point in Seattle's Laurelhurst neighborhood in the north and the northeast corner of the Madison Park neighborhood in the south. It ends at the eastern opening of the Montlake Cut of the Lake Washington Ship Canal.

When the level of Lake Washington dropped nearly nine feet in 1916 as a result of the opening of the Ship Canal, a good portion of Union Bay became dry land or marshland. The University Village shopping center and most of the University of Washington's East Campus sit on this land today.

It receives the water of Arboretum Creek, and, after construction planned for the summer of 2004 is completed, will also receive the water of Ravenna Creek via a pipeline from Ravenna Park to University Slough.