Organotin compounds are anthropogenic metal species with multiple uses as pesticides, preservatives, antifouling agents, biocides, and catalysts. Butyltins are the main organotin compounds found in biota, and the highest levels are found in marine foodstuffs. In this paper, we present the figures of merit for an in-house validated method for routine analysis of butyltins in seafood using GC inductively coupled plasma isotope dilution MS. The working range of the method spanned several orders of magnitude from 3.3-1013, 2.4-785, and 0.3-900 ng Sn/g dry weight for monobutyltin (MBT), dibutyltin (DBT), and tributyltin (TBT), respectively. The trueness of the method was evaluated by analyzing Certified Reference Materials (CRMs) ERM CRM 477 (Mussel Tissue) and NIES CRM 15 (Scallop). Recoveries, with RSD % in parentheses, were 78 (±14), 80 (±6), and 88% (±8%) for MBT, DBT, and TBT in ERM CRM 477 and 96% (±5%) for TBT in NIES CRM 15. Good agreements were found between experimental uncertainties and uncertainties predicted for single-laboratory validated methods calculated from the maximum standard measurement uncertainty function. The method has proven to be robust, and the wide range of seafood validated ensures that the method is applicable for measuring butyltins in marine tissue.