Upwelling mantle plumes can transport deep mantle materials into the shallow mantle. Therefore, identifying the lithological composition of mantle plumes in the shallow mantle is helpful for understanding the Earth's deep mantle compositions and the related geodynamic processes. Seismic tomography reveals that the root of the Réunion mantle plume is connected to the African large low-shear-velocity province in the lower mantle. This suggests that the Réunion mantle plume originates from the lower mantle. However, the lithological composition of this plume remains unclear. Ocean island basalts (OIBs) from Réunion Island were generated by partial melting of the Réunion mantle plume over the past two million years, making them ideal samples for investigating the lithological composition of the mantle plume. To explore the lithological composition of the Réunion mantle plume, this study presents whole-rock major element compositions of the Réunion OIBs, along with the chemical compositions of olivine phenocrysts within these basalts. We first estimated the primary magma compositions of the studied samples, which were found to be rich in MgO and TiO2 but deficient in SiO2, CaO, and Al2O3. When comparing the primary magma compositions with experimental melts of different mantle rocks, we found that they predominantly fall within the range of melts produced by melting of silica-deficient pyroxenite. To further evaluate this proposal, we analyzed the chemical compositions of olivine phenocrysts in the Réunion OIB samples. The results indicate that the Fo value of olivine phenocrysts ranges from 80.1 to 90.1. Notably, high-Fo olivine phenocrysts exhibit elevated Ni contents, reduced Ca and Mn contents, and high Fe/Mn ratios. These olivine compositions resemble those found in Loihi (Hawaii) OIBs, whose source contains silica-deficient pyroxenite, but they are distinctly different from olivine phenocrysts in equilibrium with peridotite-derived melts. This suggests a significant contribution of pyroxenites to the mantle source of Réunion OIBs. Therefore, both the whole-rock major elements and the compositions of olivine phenocrysts suggest that the mantle source lithology of Réunion OIBs primarily consists of silica-deficient pyroxenite. This indicates that the Réunion mantle plume contains pyroxenite components rather than being composed exclusively of peridotite.