1998–99 Washington State Cougars men's basketball team

The 1998–99 Washington State Cougars men's basketball team represented Washington State University for the 1998–99 NCAA Division I men's basketball season. Led by fifth-year head coach Kevin Eastman, the Cougars were members of the Pacific-10 Conference and played their home games on campus at Beasley Coliseum in Pullman, Washington.

The Cougars were 10–19 overall in the regular season and 4–14 in conference play, last in the standings. There was no conference tournament this season; last played in 1990, it resumed in 2002.

Shortly after the regular season ended, Eastman voluntarily resigned.