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Initiation of large igneous provinces (LIPs) and temporal and spatial variations in their chemical compositions provide unique opportunities to understand mantle dynamics. It has been recently revealed that water‐rich reservoirs in the mantle sources have played a significant role in the production of LIPs. However, the dominant causes for the chemical variations in primitive magmas of LIPs remain debated. In the Emeishan LIP (ELIP), there are several places where picrites accompany more abundant flood basalts. These picrites span a large range of Ti/Y, Sm/Yb, and TiO2/Al2O3. In this work, we have determined the water contents of picrites with Ti/Y > 500 from the ELIP, based on the water content of clinopyroxene phenocrysts and water partitioning coefficients. We have also calculated the water content of the mantle sources by applying batch and fractional partial melting models. We show that the chemical compositions of picrites in the ELIP closely correlate with the calculated water content in their mantle sources. Our model suggests that the variation in the chemical composition of the primary magmas of the ELIP both on a local and regional scale can be mainly explained by the complementary roles of relatively dry recycled pyroxenite and hydrated components in the mantle plume, without requiring catastrophic thinning of the lithospheric mantle. Our model thus implies that both the initiation and chemical variation of the ELIP are affected by water from the deep mantle. Plain Language Summary: Large igneous provinces (LIPs) represent a manifestation of the activity of a mantle plume. In most LIPs, the chemical compositions of basalts are heterogeneous and can usually be divided into high‐Ti and low‐Ti groups. This division has been used to infer differences in the mantle composition and/or partial melting conditions, which ultimately reflect the picture of the compositional and thermal structure of the deep mantle and their partial melting processes. In this study, we report that both the high‐Ti and low‐Ti type picrites of the Emeishan LIP were derived from hydrous mantle sources and such a compositional dichotomy is closely associated with the water content of the mantle sources. This case study provides a new perspective to connect the presence of volatiles in the deep mantle, the activity of deep‐seated mantle plumes, and widespread basaltic magmatism on the Earth's surface. Key Points: Both high‐Ti and low‐Ti picrites in the Emeishan large igneous province are hydrousThe major and trace element compositions of picrites are related to the water content in their mantle sourcesCompositional variations in a large igneous provinces are affected by volatiles in mantle plumes [ABSTRACT FROM AUTHOR]