Tourbillon mechanism for a watch having a turning cage mounted in pivoting manner in a first and second cage bearing lodged in fixed elements of the watch; an escapement mobile integral with the cage; and a regulating element in the form of a spiral balance capable of oscillating around a balance staff according to pulses received from the escapement mobile. The balance staff is mounted in pivoting manner in the balance bearings lodged outside the cage. The outer extremity of the spiral hairspring is fastened to the cage through a hairspring stud.