Post‐collisional ultrapotassic magmatism received special attention because it is an important indicator for mantle geodynamics in a collisional orogen. So far, no mantle‐derived magmatism has been identified within the Himalayan belt, hindering our understanding of the architecture and thermal state of the deep‐seated lithosphere in this area during the orogenesis. Here, we report the first discovery of mantle‐derived ultrapotassic rock in the Himalayan region. The middle Miocene (ca. 13 Ma) lamprophyre dike intruded the Tethyan Himalaya sedimentary cover of the Ramba dome on the shoulder of the Yadong‐Gulu rift. Geochemical analyses suggest that the ultrapotassic melt was derived from the subcontinental lithospheric mantle of the Indian plate, within a relatively shallow depth in the spinel stability field. The ultrapotassic magmatism indicates asthenospheric upwelling in the middle Miocene, a response to the mantle unrooting process beneath the Himalayan orogen.