The 1998–99 Oklahoma State Cowboys basketball team represented Oklahoma State University as a member of the Big 12 Conference during the 1998–99 NCAA Division I men's basketball season.
They were led by 9th-year head coach Eddie Sutton and played their home games at Gallagher-Iba Arena in Stillwater, Oklahoma.
The Cowboys lost to Kansas in the championship game of the Big 12 tournament.
The team received an at-large bid to the NCAA tournament as the No.
8 seed Syracuse in the opening round before losing to No.