The Arabia‐Eurasia continental collision created the Caucasus Mountains and the Anatolian and Iranian plateaus. Between the two plateaus, the Armenian Highland features young Holocene‐aged volcanoes. In this study, the P‐wave receiver functions from a new seismic array reveal a thick crust (up to ~52 km thick) beneath the Central Greater Caucasus and an unusually thin crust (32–35 km thick) beneath the northwestern part of Armenia near the Aragats stratovolcano and Gegham volcanic ridge formed by Pleistocene to Holocene monogenetic cinder cones. The average crustal V_P/V_S ratio in the Armenian Highland is anomalously high (≥1.9), with the highest value approaching 2.1 under the Gegham Ridge. Such high V_P/V_S ratios cannot be explained by an overall mafic crustal composition. Instead, the presence of partial melts is inferred in the lower crust based on the depth of the low‐velocity structure obtained by inversion of receiver function waveforms. Our study suggests that the postcollisional volcanism was potentially facilitated by the small‐scale removal of lithospheric mantle, resulting in a localized thinner crust balanced by thermal buoyancy and dynamic flow in the uppermost mantle beneath the Armenian Highland.