Tetramine [tetramethylammonium ion, (CH₃)₄N⁺] is a toxic substance found in marine predatory gastropods causing food poisoning when consumed. Despite a considerably high consumption of carnivorous marine gastropods in Korea, studies in tetramine are limited in Korea. This study analyzed tetramine content in eight carnivorous and four herbivorous marine gastropod species obtained from the east coast of Korea using liquid chromatography–tandem mass spectrometry (LC–MS/MS). Method validation showed good linearity (r² over 0.999) with a wide linear range (0.1–50 mg/kg), low method detection limit (0.008–0.011 mg/kg), and low method quantitation limit (0.025–0.033 mg/kg) in low-fat/high-fat matrices. The intraday (n = 5) and interday (3 days, n = 15) accuracy and precision results met the CODEX guidelines. The salivary glands of Neptunea showed higher tetramine levels than the other species, and the average tetramine content of the salivary glands was higher in the order of Neptunea cumingii (7,610 mg/kg), Neptunea arthritica (4,950 mg/kg), and Neptunea eulimata (2,710 mg/kg). Tetramine content was higher in the salivary gland (up to 16,700 mg/kg) than in other tissues. Tetramine was detected only in the predatory species in the genus Neptunea, not in herbivorous gastropod species. To prevent tetramine food poisoning, the salivary glands of the predatory gastropods should be removed when they are cooked and served.