The invention relates to a device for guiding a sliding window (28) in a door (2) of a motor vehicle, which comprises a guide rail (5) and at least one guide runner (4) secured to the inner face of said sliding window (28), said guide rail (5) being provided with a cylindrical longitudinal cavity (52), said guide runner (4) comprising at least one guide head (42) fitted in said cavity (52). Said guide runner (4) is rigid and the guide head (42) thereof comprises at least three flat parts (420) that extend radially from an axis (421) of the runner head, and said guide rail (5) surrounds said guide head (42) such that the free ends (422) of each flat part (420) of the guide head (42) bear permanently against the inner wall of said longitudinal cavity (52).