Ciomadul is the youngest volcano in the Carpathian‐Pannonian Region, Eastern‐Central Europe, which last erupted 30 ka. This volcano is considered to be inactive, however, combined evidence from petrologic and magnetotelluric data, as well as seismic tomography studies, suggests the existence of a subvolcanic crystal mush with variable melt content. The volcanic area is characterized by high CO2 gas output rate, with a minimum of 8.7 × 103 t/year. We investigated 31 gas emissions at Ciomadul to constrain the origin of the volatiles. The δ13C–CO2 and 3He/4He compositions suggest the outgassing of a significant component of mantle‐derived fluids. The He isotope signature in the outgassing fluids (up to 3.10 Ra) is lower than the values in the peridotite xenoliths of the nearby alkaline basalt volcanic field (R/Ra 5.95 Ra ± 0.01), which are representative of a continental lithospheric mantle and significantly lower than MORB values. Considering the chemical characteristics of the Ciomadul dacite, including trace element and Sr–Nd and O isotope compositions, an upper crustal contamination is less probable, whereas the primary magmas could have been derived from an enriched mantle source. The low He isotopic ratios could indicate a strongly metasomatized mantle lithosphere. This could be due to infiltration of subduction‐related fluids and postmetasomatic ingrowth of radiogenic He. The metasomatic fluids are inferred to have contained subducted carbonate material resulting in a heavier carbon isotope composition (δ13C is in the range of −1.4‰ to −4.6‰) and an increase of CO2/3He ratio. Our study shows the magmatic contribution to the emitted gases. Plain Language Summary Determining the fluxes and composition of gases in active and dormant volcanoes could help to constrain their origin. Ciomadul is the youngest volcano of the Carpathian‐Pannonian Region, Eastern‐Central Europe, where the last eruption occurred 30 ka. Its eruption chronology is punctuated by long quiescence periods (even >100 kyr) separating the active phases; therefore, the long dormancy since the last eruption (30 ka) does not unambiguously indicate inactivity. Knowing if melt‐bearing magma resides in the crust is fundamental to evaluate the nature of the volcano. Isotopic compositions of helium (3He/4He) and carbon (δ13CCO2) are important tools for the study of the origin of the gases. We show that the isotope variation of the emitted gases suggests a metasomatized lithospheric mantle origin for the primary magmas. This is consistent with a degassing deep magma body existing beneath Ciomadul, and this long‐dormant volcano cannot be considered as extinct. Key Points CO2 emissions at Ciomadul, Eastern‐Central Europe, suggest a still‐active plumbing system beneath the volcano in spite of long dormancy The CO2 and He isotope compositions provide evidence for significant contribution of magma‐derived volatiles, up to 80% Isotopic signatures of gases indicate that primary magmas could have derived from a mantle source modified by subduction‐related fluids