The STS Kaisei (海星), meaning “Sea Star” in the Japanese language, is a steel-hulled brigantine designed by Polish naval architect Ryszard Langer.

After re-rigging it is now a two-masted vessel, square rigged on the foremast, with fore-and-aft sails on the mainmast.

Originally commissioned by the Polish Yachting Association as the Zew, she was acquired by the Sail Training Association of Japan (STAJ) and delivered to Japan via the Panama Canal in 1993 following a 16-month maiden voyage under the United Nations flag in which she saw 100-knot (190 km/h) winds off the coast of Scotland and participated in the American Tall Ship Celebration in 1992.

Ocean Voyages Institute, a California 501(c) 3 organization, currently operates the vessel Kaisei and she served as the figurehead and expedition ship of "Project Kaisei" which is an international program dedicated to implementing solutions to the problem of marine debris.

She returned again in 2010 to the North Pacific Gyre and also took part in a variety of educational and awareness events.