On January 4, 2005, Kōyagi, along with the towns of Iōjima, Nomozaki, Sanwa, Sotome and Takashima (all from Nishisonogi District), was merged into the expanded city of Nagasaki and no longer exists as an independent municipality.
A park and viewpoint is located in the centre of Koyagi from which fine views of Nagasaki city and the peninsular can be seen.
In 1936 Toyosaku Kawanami bought the Matsuo Shipyard in Kōyagi and transformed it into one of the largest in the Far East.
During World War II, a POW camp, Fukuoka #2 was located in Kōyagi and the prisoners were forced to work in brutal conditions in the factories.
⟨⟩⟩ In 1967, Mitsubishi Heavy Industries acquired the old shipyard facility and started a process of development to create the largest dock in Japan.