Mantle-derived ultrapotassic rocks (K₂O/Na₂O > 2, K₂O > 3 wt%, and MgO > 3 wt%) are crucial to reveal crust-mantle interactions. Here, we present petrographic compositions, whole-rock geochemical and potassium (K) isotopic compositions of Baoji ultrapotassic rocks from North Qinling Orogen, China, to trace their origin. The high Ba/La, Ba/Th, Ba/Rb and low Rb/Sr ratios of the investigated ultrapotassic rocks, together with partial melting modelling, reveal a garnet-facies amphibole lherzolite mantle source modified by slab-derived fluids. In particular, the δ⁴¹K values of the ultrapotassic rocks (−0.57‰ to −0.06‰) are higher than that of the depleted mantle and display positive correlations with K/Th, Ba/Th and Ba/Rb ratios, which indicate that slab-derived fluids have contributed heavy K isotopic signatures to the mantle source from which the ultrapotassic rocks were derived. Collectively, the data demonstrate that slab-derived fluids, or supercritical fluids play important roles in the generation of post-collisional ultrapotassic rocks and K isotopes have the ability to trace the deep processes in subduction zones.