A jet diffuser with a quick-change nozzle for use in irrigation systems includes an upper supply portion with a main body that has an upper tubular portion with a longitudinal axis, which is designed to be connected to a drop line and a lower hollow portion having a central cavity and a side wall, in which a lateral opening is formed, a nozzle adapted to be radially inserted into the cavity through the lateral opening in substantially coaxial and facing relationship to the upper tubular portion to direct a liquid jet axially downwards, and a quick-connect system associated with the nozzle for securing it within the main body and including cam and follower drive elements operating on the nozzle to move it and push it axially upwards into substantial contact and sealing relationship to the lower edge of the upper tubular portion, possibly with no seal interposed therebetween.