Yūji Koseki, Japanese composer (d. 1989)
Yūji Koseki (古関 裕而, Koseki Yūji, August 11, 1909 – August 18, 1989) was a Japanese ryūkōka, gunka, march, fight song and film score composer. His real name was also Yūji Koseki, but its kanji was 古關 勇治.

Koseki entered Nippon Columbia in 1930. He composed Hanshin Tigers' song "Rokko Oroshi" in 1936. His famous military song titled "Roei no Uta" (露営の歌, lit. "The Song of The Camp") was released in 1937. Famous songs composed by him included "The Bells of Nagasaki" and "Mothra's song". Ichiro Fujiyama sang "The Bells of Nagasaki" in 1949. "Mothra's song", sung by The Peanuts, was used in the 1961 movie Mothra. "Olympic March" in 1964. He also arranged "Olympic Hymn" for Orchestra.