An adjustable handlebar mounting device includes a frame mounting member, a front pivot member, a rear pivot member, and a releasable fastening unit. The frame mounting member is formed with front and rear lug units. The front pivot member has a front lower section connected pivotally to the front lug unit, a front upper section to be connected to a handlebar, and an intermediate coupling part. The rear pivot member has a rear lower section connected pivotally to the rear lug unit, and a rear upper section. The coupling part and the rear upper section are respectively formed with a slot unit and a hole unit for extension of the fastening unit therethrough. The fastening unit is operable so as to press the coupling part and the rear upper section firmly against each other for supporting the handlebar.