In this study, new constraints on post-collisional granitoid magmatism in the Internal Dinarides are provided through petrographic, mineral-chemical, zircon U–Pb geochronological, and zircon Lu–Hf isotopic analyses of the monzogranite intrusion exposed along the southwestern flank of the Kosmaj Mts. in central Serbia. The intrusion consists of a calc-alkaline monzogranite composed mainly of orthoclase, plagioclase, and quartz, with subordinate phlogopite and magnesiohornblende. Amphibole–plagioclase thermobarometry indicates crystallization at 774–831 °C and ~160–260 MPa, consistent with shallow upper-crustal levels prior to or during final emplacement. Zircon U–Pb data yield a concordia age of 24.92 ± 0.31 Myr, constraining magmatism to the Late Oligocene. Zircon Lu–Hf isotopic compositions (εHf(t) = −1.2 to +7.0) point to a predominantly juvenile magma source with limited crustal involvement. Based on its age, petrographic characteristics, and zircon Lu–Hf isotopic composition, the Kosmaj monzogranite is assigned to the Oligocene–Early Miocene post-collisional I-type granitoid suite of the Internal Dinarides.