A positioning device includes a mount secured on a seat tube, and left and right joint members having joint surfaces which abut against the mount, and gripping cavities which mate with clamping cavities of left and right clamping members to permit lengthwise segments of left and right anchoring bars of a seat to be clamped in left and right bar-engaging zones, respectively. When the joint surfaces switch places or when the left and right clamping members are shifted, the bar-engaging zones are displaced from a first position to be proximate to corresponding ones of front and rear ends of the anchoring bars, to a second position to be proximate to the other corresponding ones of the front and rear ends of the anchoring bars, thereby enabling the seat to be adjusted to a desired position relative to the seat tube.