| Keijuro Matsui | |
| Position: | Guard |
| Height Ft: | 6 |
| Height In: | 2 |
| Weight Lb: | 183 |
| Team: | Kagawa Five Arrows |
| League: | B.League |
| Number: | 16 |
| Birth Date: | 16 October 1985 |
| Birth Place: | Suginami, Tokyo, Japan |
| High School: | Montrose Christian School (Rockville, Maryland) |
| College: | Columbia (2005–2009) |
| Draft Year: | 2009 |
| Career Start: | 2009 |
| Years1: | 2009–2010 |
| Team1: | Rera Kamuy Hokkaido |
| Years2: | 2010–2011 |
| Team2: | Hitachi SunRockers |
| Years3: | 2011–2017 |
| Team3: | Toyota Alvark |
| Years4: | 2017–2019 |
| Team4: | SeaHorses Mikawa |
| Years5: | 2019–2021 |
| Team5: | Kyoto Hannaryz |
| Years6: | 2021–2023 |
| Team6: | Toyama Grouses |
| Years7: | 2023–present |
| Team7: | Kagawa Five Arrows |
Keijuro Matsui (popularly known as KJ) (born October 16, 1985) is a Japanese professional basketball player for the Kagawa Five Arrows of the B.League. He is a former guard for the Columbia Lions basketball team at Columbia University and the first Japanese native basketball player in NCAA Division I history. Matsui also played in the 2005 Nike Hoop Summit for the World Select Team, only the second Japanese to do so. In the summit he scored seven points in 11 minutes. Following his collegiate career, he relocated to his native country, where he has played ever since.
| † | Denotes seasons in which Matsui won an championship |
| Led the league | |