Kazuo "Kaz" Matsui (松井 稼頭央, Matsui Kazuo?) (born October 23, 1975 in Higashiōsaka, Osaka, Japan) is a Japanese Major League Baseball second baseman for the Houston Astros. Matsui is a switch-hitter. Matsui is not related to Hideki Matsui, the designated hitter for the New York Yankees.

Matsui signed with the New York Mets on December 17, 2003, becoming the first Japanese infielder to sign with an MLB team.

Matsui graduated from the PL Academy Senior High School in Osaka, a school nationally renowned for its baseball program. The only appearance Matsui made at the National High School Baseball Championship Tournament was in his second year at PL Academy. Though Matsui was considered to be the PL Academy's ace starting pitcher, injuries limited him to playing only in the quarter-final game, where he allowed two runs during 2 2/3 innings.