A seat position-adjusting device is used to connect adjustably a seat to a leg unit of a highchair, and includes a tube component, a slide seat and a locking unit. The tube component is sleeved on the leg unit, and is formed with positioning holes aligned along a tube axis. The slide seat is sleeved on the tube component, and is connected to the seat. The locking unit includes a latch disposed in the slide seat, pivoted to the slide seat, and pivotable between locking and unlocking positions to engage and disengage a selected positioning hole to thereby selectively lock and unlock the slide seat relative to the tube component.