A guide device for a sliding door includes a base seat and a pair of side wall units. The base seat is formed with at least one guide groove. Each of the side wall units includes a rigid sidewall piece coupled to the base seat, movable relative to the base seat, and has at least one guide leg that engages slidably with the at least one guide groove. The rigid sidewall pieces of the side wall units define a gap therebetween. Each of the side wall units further includes a positioning element mounted to one of the rigid sidewall piece and the base seat and abutting releasably against the other one of the rigid sidewall piece and the base seat for locking the rigid sidewall piece to the base seat and fixing a width of the gap.