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.
Portage Bay was named in 1913 because of the portage across the Montlake Isthmus 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 State Route 520 from Montlake to Capitol Hill.
In 1861, landowner Harvey Pike cut the first ditch connecting Lake Washington's Union Bay and Lake Union's Portage Bay, but gave up and deeded his land to the Lake Washington Ship Canal Company, which built a transiting rail line for portaging goods between the lakes. This rail line continued use until 1878. In 1883, David Denny and Thomas Burke had a canal built for floating logs.
Cheshiahud lived and carved canoes on the shores of Portage Bay for many years from 1885. In 1909, the Pay Streak, a vaudeville and sideshows section of the Alaska-Yukon-Pacific Exhibition, ended in a gondola ramp and esplanade on the north shores of Portage Bay. 
For 14 years following World War II, the SV Fantome was stranded in Portage Bay pending payment to King County of back taxes.
The small residential neighborhood of Portage Bay, Seattle, is located on the southwestern shore of the bay.
- Majors, Harry M. (1975). Exploring Washington. Van Winkle Publishing Co. p. 78. ISBN 978-0-918664-00-6.
- Long, Priscilla. "Montlake log canal first connects Seattle's Union and Portage bays in 1861". HistoryLink.org Essay 3404. Retrieved 29 April 2011.
- Long, Priscilla (24). "Chinese laborers dig second Montlake Cut between Union Bay and Portage Bay in 1883". HistoryLink.org Essay 3349. HistoryLink.org. Retrieved 29 April 2011.
- "Alaska-Yukon-Pacific Exposition, 1909 -- A Slideshow of Seattle's First World's Fair". HistoryLink.org Essay 7082. HistoryLink.org. Retrieved 29 April 2011.
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