Shale instability due to hydration and swelling is an unavoidable problem during hydraulic fracturing or drilling with water-based fluids. In this study, 1-butyl-3-methylimidazolium chloride (BMIMCl) an ionic liquid was utilized as a clay swelling inhibitor. The performance of BMIMCl was evaluated by the bentonite plate soaking test and linear swelling test. FT-IR and zeta potential test was done to explain the swelling inhibition mechanisms. Moreover, XRD test was conducted for the characterization of bentonite (swelling clay). 2 wt. BMIMCl reduced the swelling rate of bentonite by 19.38 while the conventional and mostly used inhibitor, 3 wt. KCl reduced 13.84 compared with distilled water. These results showed that BMIMCl has better inhibitive properties than the most used clay stabilizer, KCl. This inhibitor showed better performance because of the presence of a hydrophilic head and hydrophobic tail in its structure. The hydrophilic positive head helped it to get adsorbed on the bentonite surface, forming hydrogen bonds, and reduced the surface negative charge. On the other side, the butyl chain may be made a hydrophobic shield that prevented water from entering into the interlayer space. The ability of BMIMCl to form bonds with bentonite and water was confirmed by FT-IR analysis. Therefore, BMIMCl has the potential to be an effective shale swelling inhibitor during drilling and hydraulic fracturing operations in the water-sensitive shale formation. © Published under licence by IOP Publishing Ltd.