A critical component of the biogeochemical cycle of mercury (Hg) is the transformation of inorganic Hg to neurotoxic monomethylmercury (CH3Hg). Humans are exposed to CH3Hg by consuming marine fish, yet the origin of CH3Hg in fish is a topic of debate. The carbon stable isotopic composition (δ(13)C) embedded in the methyl group of CH3Hg remains unexplored. This new isotopic information at the molecular level is thought to represent a new proxy to trace the carbon source at the origin of CH3Hg. Here, we present a compound-specific stable isotope analysis (CSIA) technique for the determination of the δ(13)C value of CH3Hg in biological samples by gas chromatography combustion isotope ratio mass spectrometry analysis (GC-C-IRMS). The method consists first of calibrating a CH3Hg standard solution for δ(13)C CSIA. This was achieved by comparing three independent approaches consisting of the derivatization and halogenation of the CH3Hg standard solution. The determination of δ(13)C(CH3Hg) values on natural biological samples was performed by combining a CH3Hg selective extraction, purification, and halogenation followed by GC-C-IRMS analysis. Reference δ(13)C values were established for a tuna fish certified material (ERM-CE464) originating from the Adriatic Sea (δ(13)C(CH3Hg) = -22.1 ± 1.5‰, ± 2 SD). This value is similar to the δ(13)C value of marine algal-derived particulate organic carbon (δ(13)CPOC = -21‰).