Basil seed, containing anionic heteropolysaccharides in its outer pericarp, swells as gelatinous hydrocolloid when soaked in water. In this study, basil seed powder (BSP) was used as a multifunctional additive for water-based drilling fluids. The chemical composition, water absorbency, rheological properties of aqueous suspension of BSP were tested. The effect of BSP on the rheological and filtration of bentonite-based drilling fluid before and after thermal aging was investigated. The inhibition characteristics were evaluated by linear swelling, shale cuttings dispersion and shale immersion test. Lubricity improvement by BSP was measured with extreme pressure lubricity test. The results revealed that incorporation of BSP into bentonite suspension improved rheological and filtration properties effectively after thermal aging of 120 °C. BSP exhibited superior inhibitive capacity to xanthan and synergistic effect with KCl. BSP could reduce friction by forming hydration layer. The nanoscale three-dimensional network structures enable BSP to maintain high water retention and absorb strongly on bentonite and metal surface, contributing to enhanced rheology, filtration, inhibition and lubrication properties. The versatile characteristic of BSP, as well as biodegradation makes it a promising additive using in high performance water-based drilling fluid and a potential alternative to conventional synthetic polymers.