Amir Khadir says it's time to give someone else a chance after four years at the helm.
He will keep his seat in the legislature.
Quebec solidaire traditionally has two spokespersons, one for the legislature and one for outside the legislature.
A new spokesperson will be elected at a party convention at the end of the month.
Francoise David, who was elected on Sept. 4, will continue as the party's other spokesperson.