A drive transmission system for a vessel propulsion device 1 provided with a clutch operating device that can easily be installed comprises a shift slider 40 axially slidable within a propeller shaft 9 to move a clutch gear 30 through a clutch shifter pin 38 inserted through the propeller shaft; and an operating member capable of operating the shift slider from above a gear case 5; the operating member includes: a shift fork 50 engaged with a fork engaging portion 35c of the shift slider, to move the shift slider; a guide member 59 supporting the shift fork movable in a specific direction; a rotary rod member 55 engaged with a rod engaging section 50b of the shift fork to move the shift fork; and a support member 51 drivably supporting the rod member and fixedly supporting the guide member, thereby sub-assembling the rod member and the guide member to transmit the operation of the rod member to the shift fork; the shift fork is so assembled to be engaged with the fork engaging section of the shift slider by fitting the support member from above in a specific fitting section in the gear case, thereby transmitting the movement of the rod member to the shift slider.