The levels of organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) were assessed in horse mackerel (Trachurus sp.) samples collected monthly between October 2010 and October 2011 from four regions (Bosphorus, Bandırma, Gelibolu, Tekirdağ) in the Marmara Sea. α-Endosulfan and heptachlor were not detected in any fish samples. The annual average OCP levels among the regions ranged between 0.09–12.2 ng g−1 (wet weight), 0.8–168 (lipid weight), while the annual average PCB levels among the regions ranged between 0.55–4.98 ng g−1 (wet weight), and 7.01–116 (lipid weight), respectively. Total-DDT, total-HCH, PCB 138 and PCB 153 were the most predominant contaminants in the fish samples. There was no relation between lipid content of horse mackerel samples and residues of the contaminants. OCP and PCB levels detected in all of the fish samples were below the limits recommended by the EU Commission (2011), Food and Agricultural Organization of the United Nations (1983), U.S. Food and Drug Administration (2011) and Turkish Food Codex (2011).