Given our laboratory interest in IFITM3 S-fatty-acylation and antiviral activity, we sought to directly characterize fatty acids that are covalently attached to the Cys residues of IFITM3. IFITM3 comprises two S-fatty-acylation sites (C71 and C72) in proximity of a intramembrane domain, and another site adjacent to a transmembrane domain (C105). We directly identified the S-fatty-acylation sites of IFITM3 and further demonstrated that the highly conserved Cys residues are primarily modified by palmitic acid.