Issei Koga (古賀 一成, Koga Issei, born July 30, 1947) is a Japanese politician of the Democratic Party of Japan, a member of the House of Representatives in the Diet (national legislature). A native of Yanagawa, Fukuoka and graduate of the University of Tokyo, he was a bureaucrat at the Ministry of Construction from 1971 to 1988. He was elected to the House of Representatives for the first time in 1990.