The Montefiascone Volcanic Complex belongs to the Vulsini Mts. volcanic district, one of the several leucite-bearing ultrapotassic Pleistocene volcanoes of the Roman Province (Central Italy). It is characterized by abundant pyroclastic successions and minor lava flows emplaced during a caldera formation phase occurred ∼0.3–0.2 Ma. The volcanic products span from leucite-basanites and leucite-tephrites to rarer leucitites and melilitites, occasionally associated with very rare kalsilite melilitolite ejecta. Here we report for the first time the presence of primary kalsilite in two lava flows in Forcinella and Feltricci localities, identifying a kamafugitic component among the Montefiascone volcanics. Detailed petrographic, SEM and EPMA studies on ten samples from the two localities revealed, in addition to kalsilite, variable amounts of clinopyroxene, melilite, leucite and nepheline plus rarer olivine coupled with carbonate-bearing segregation pockets in the groundmass. Geochemical and isotopic data confirm the exotic nature of the analysed rocks, showing ultrabasic/basic (SiO2 = 42.15–45.94 wt%) and ultrapotassic (K2O = 6.17–8.48 wt%; K2O/Na2O = 5.3–8.7) compositions, as well as strongly radiogenic 87Sr/86Sr (0.71038–0.711068), poorly radiogenic 143Nd/144Nd (0.512057.512091) and uniform middle radiogenic 206Pb/204Pb (18.76–18.78). Boron isotopes (δ11B ranges from 7.36 to 8.82‰) almost overlaps the typical MORB-OIB range values. These geochemical features recall those of the nearby kamafugite rocks from the Intra-Apennine Province. Major oxide and trace element modelling support fractionation links among the Italian kamafugite types. The overall geochemical and isotopic signatures speak for a common strongly subduction-metasomatized and orthopyroxene-free lithospheric mantle source. As already proposed for some nearby kamafugite centres in central Italy, limestone assimilation during magma ascent is a viable option to explain the mineralogical and whole-rock composition of Montefiascone lavas (i.e., the presence of carbonate plagues in the lava groundmass). This is particularly relevant considering the thick sedimentary sequence pierced by the limited volume of the upwelling magma. On the other hand, many other key features (such as the general lack of crustal xenoliths, the overall incompatible element fractionation and the radiogenic Sr isotopic ratios) clearly require also derivation from subduction-modified mantle sources. •New kamafugite occurrence in Pleistocene leucite-rich ultrapotassic volcanics, Italy.•Whole-rock data point to a mantle source rich in phlogopite, clinopyroxene, carbonate.•Mass balance models support fractional crystallization links among kamafugite melts.