BackgroundMolar incisor hypomineralisation (MIH) is a problematic condition with several characteristics for which infiltrant resins could theoretically improve clinical outcomes.AimTo investigate whether caries infiltrant resin can penetrate MIH‐affected enamel.DesignMolar incisor hypomineralisation lesions (n = 21) were infiltrated using either the standard protocol or with the addition of a sodium hypochlorite (NaOCl) irrigation step. Lesions were sectioned and examined microscopically for infiltrant penetration before undergoing Vickers hardness testing. The surfaces of several lesions were also examined using scanning electron microscopy (SEM).ResultsInfiltrant resin could penetrate MIH lesions; however, the pattern was erratic. Two lesions were confined to inner enamel, and no infiltration occurred. On average, the resin penetrated to a depth of 0.67 ± 0.39 mm and 23.1 ± 15.2% of the area of the lesion. Microhardness increased in areas of resin penetration by 1.0 ± 0.7 GPa representing a proportional increase of 2.2 ± 2.5 times. There were no significant differences in results based on either the infiltration protocol or the type of MIH lesion.ConclusionCaries infiltrant resin is capable of penetrating MIH enamel lesions; however, the pattern, extent, and change in hardness produced are currently unpredictable.