A positioning device for the multi-section slide track assembly includes an inner slide track, an intermediate slide track, and an outer slide track. The positioning device consists of a positioning member and an actuating member pivot-connected thereto. The positioning member provides with at least one engaging end while the actuating member providing with at least one bent guiding edge and a push button. The engaging end is adapted to engage with an oblique protrusion of the outer slide track for positioning the intermediate slide track. A return movement of the inner slide track can actuate the bent guiding edge for automatically unlocking the engagement of the intermediate slide track with the outer slide track. Alternatively, a user can manually press the push button to unlock the engagement of the intermediate slide track with the outer slide track.