Oceanic plagiogranites are directly or indirectly related to ophiolites. A small plagiogranite vein has been discovered in the No. III mafic–ultramafic complex of the Xiarihamu ophiolite from the eastern margin of Qiman Tagh, East Kunlun Orogen, northwest China. This plagiogranite yielded a weighted mean zircon U–Pb age of 435.7 ± 4.3 Ma (MSWD = 0.073) and is characterized by high SiO2 (73.08–76.24 wt.%) and Na2O (6.42–8.34 wt.%) contents and high Na2O/K2O ratios (3.06–12.36) with metaluminous and calc-alkaline affinity. It is enriched in Th and U, depleted in Nb, Sr, Ti, and Ba, and has low ΣREE contents (7.64–11.35 ppm), negative Eu anomalies (δEu = 0.31–0.71), and light REE enrichment. These features are typical of oceanic plagiogranites. Whole-rock Sr–Nd isotopic data for the plagiogranite show obvious EMII mantle characteristics. The petrographic, geochemical, and Sr–Nd isotopic characteristics suggest the plagiogranite is part of an ophiolite formed by partial melting of oceanic crust in the presence of hydrothermal fluids. Given this, other parts of the Xiarihamu ophiolite and other ophiolites in Qiman Tagh area, we propose the plagiogranite formed in a back-arc basin setting in the early Silurian, probably the youngest known intrusion related to the Proto-Tethys Ocean.