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A fast and cost-effective procedure based on Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) for the selective determination of methylmercury (MeHg) is proposed and validated for fish tissue analysis. Selectivity for MeHg is achieved by simply inserting a strong anion exchange resin to block inorganic mercury as negatively charged chloride species, leaving MeHg unretained. The procedure features a 15 min extraction time followed by a 100 s analysis time achieving a limit of detection of 1.6 μg kg−1 on solid samples. The effect of the solution composition and inorganic mercury concentration were extensively studied to fully assess the selectivity of the procedure: Hg(II):MeHg ratios up to 50 are tolerated and cause systematic errors lower than 15%. The entire procedure was successfully validated by standard reference material from the marine food web, namely fish muscle and liver plus zooplankton. The method was finally applied to the detection of MeHg in the marine trophic web of Djibouti (Gulf of Aden): a trophic magnification factor of 13.5 proved the high risk associated with the biomagnification of methylmercury. [Display omitted] • A highly selective ICP-MS method for methylmercury determination is presented. • A short, homemade anion-exchange column enables the online trapping of Hg(II). • The implementation of the separation system to ICP-MS is simple and cost-effective. • The analysis requires 100s and is compatible with a fast extraction strategy. • The method was validated by standard reference materials from the marine food web. [ABSTRACT FROM AUTHOR]