The physicochemical aspect of wellbore stability is linked to the interaction between the drilling fluid and rock (e.g., shale). By far, most of the surfactants utilized in the oil industry are identified with environmental drawbacks. This paper appraises for the first time the functionality of a novel environmentally-friendly bio-based surfactant, Seidlitzia Rosmarinus leaf and stem extract (SRLSE), in water-based drilling fluids (WBDF) for inhibiting shale hydration. The swelling inhibitive property of SRLSE was thoroughly evaluated using several experiments including clay inhibition, cuttings dispersion, filtration, particle size measurement, scanning electron microscopy (SEM) and thermal gravimetric analysis (TGA). The findings implied that as opposed to deionized water, the aqueous solution of SRLSE indicated fairly low rheological profiles which resulted in a greater degree of montmorillonite (MMT, an active mineral in shale) loading (22.5 mass%). Addition of 3 mass% SRLSE and 8 mass% SRLSE to deionized water resulted in a shale cuttings recovery improvement of 20.8% and 40.5%, respectively. MMT in deionized water gave a fluid loss of 26 ml. However, it exhibited a poor fluid loss control in the aqueous solution of SRLSE so that the fluid loss volume increased up to 73 ml. As opposed to deionized water, the particle size of MMT in SRLSE aqueous solution rose extremely. According to TGA, the modified MMT in the aqueous solution of SRLSE imposed 2.88% less water content with respect to the modified MMT in deionized water. From SEM observations, MMT in SRLSE aqueous solution exhibited larger aggregates than deionized water. In addition to the aforementioned results, compatibility tests proved that SRLSE is well-compatible with typical WBDF additives. All the experimental results implied that SRLSE can be considered a capable shale inhibitor in WBDF. The interaction between the hydrophilic tail of saponin (a dominant constituent of SRLSE) and MMT's surfaces which forms a hydrophobic shell is perhaps the leading inhibition mechanism for SRLSE. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.