| Seiji Koga 古賀 誠史 | |
| Fullname: | Seiji Koga |
| Birth Date: | 7 August 1979 |
| Birth Place: | Okawa, Fukuoka, Japan |
| Position: | Midfielder |
| Youthyears1: | 1995–1997 |
| Youthclubs1: | Higashi Fukuoka High School |
| Years1: | 1998–2001 |
| Clubs1: | Yokohama F. Marinos |
| Caps1: | 9 |
| Goals1: | 0 |
| Years2: | 2002–2007 |
| Clubs2: | Avispa Fukuoka |
| Caps2: | 134 |
| Goals2: | 23 |
| Years3: | 2007–2009 |
| Clubs3: | Vissel Kobe |
| Caps3: | 34 |
| Goals3: | 3 |
| Years4: | 2010–2012 |
| Clubs4: | SC Sagamihara |
| Totalcaps: | 177 |
| Totalgoals: | 26 |
is a former Japanese football player. His brother Masahiro Koga is also former footballer.
Koga was born in Okawa on August 7, 1979. After graduating from high school, he joined the J1 League club Yokohama Marinos (later Yokohama F. Marinos) in 1998. Although, he played several matches as an offensive midfielder every season, he did not play in many matches. In 2002, he moved to the J2 League club Avispa Fukuoka based in his local area. He played many matches as left offensive midfielder and the club was promoted to J1 in 2006. However the club was relegated to J2 in a year and his opportunity to play decreased in 2007. He became a regular player and played often until May 2008. After that, he could not play due to an injury. Although he came back in July 2009, he was released from the club at the end of the 2009 season. In May 2010, he joined the Prefectural Leagues club SC Sagamihara. He played often and the club was promoted to the Regional Leagues in 2011. He retired at the end of the 2012 season.
| Club performance | League | Cup | League Cup | Total | |||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Season | Club | League | Apps | Goals | Apps | Goals | Apps | Goals | Apps | Goals | |
| Japan | League | Emperor's Cup | J.League Cup | Total | |||||||
| 1998 | Yokohama Marinos | J1 League | 3 | 0 | 0 | 0 | 0 | 0 | 3 | 0 | |
| 1999 | Yokohama F. Marinos | J1 League | 2 | 0 | 0 | 0 | 0 | 0 | 2 | 0 | |
| 2000 | 0 | 0 | 0 | 0 | 2 | 0 | 2 | 0 | |||
| 2001 | 4 | 0 | 0 | 0 | 1 | 0 | 5 | 0 | |||
| 2002 | Avispa Fukuoka | J2 League | 17 | 1 | 0 | 0 | colspan="2" | - | 17 | 1 | |
| 2003 | 25 | 5 | 3 | 2 | colspan="2" | - | 28 | 7 | |||
| 2004 | 33 | 9 | 0 | 0 | colspan="2" | - | 33 | 9 | |||
| 2005 | 30 | 5 | 0 | 0 | colspan="2" | - | 30 | 5 | |||
| 2006 | J1 League | 21 | 1 | 2 | 0 | 4 | 0 | 27 | 1 | ||
| 2007 | J2 League | 8 | 2 | 0 | 0 | colspan="2" | - | 8 | 2 | ||
| 2007 | Vissel Kobe | J1 League | 11 | 3 | 2 | 0 | 0 | 0 | 13 | 3 | |
| 2008 | 13 | 0 | 3 | 0 | 0 | 0 | 16 | 0 | |||
| 2009 | 10 | 0 | 3 | 0 | 0 | 0 | 13 | 0 | |||
| Total | 177 | 26 | 13 | 2 | 7 | 0 | 197 | 28 | |||