Portage Bay is an arm of Seattle, Washington's Lake Union and is part of the Lake Washington Ship Canal. Its western limit can be said to be the Ship Canal Bridge, which carries Interstate 5 over the water; North Passage Point Park and South Passage Point Park sit on opposite shores between the freeway's pillars. Its eastern limit is the entrance to the Montlake Cut.

Named after the portage that used to be necessary to move logs from Union Bay to Lake Union before the construction of the Ship Canal, the bay is home to two yacht clubs, the Seattle and the Queen City, and many houseboats, as well as the Northwest Fisheries Science Center of the National Marine Fisheries Service and the University of Washington's College of Ocean and Fishery Science. It is spanned by the University Bridge, which carries Eastlake Avenue from Eastlake to the University District, and by the Portage Bay Viaduct, which carries Washington State Route 520 from Montlake to Capitol Hill.