Background: formaldehyde contaminants have been detected in seafood all the time in different areas formaldehyde is a dangerous chemical. If food is contaminated, it can be detrimental to health and various systems in the body. Objective: To detect formaldehyde contaminants in seafood in Bangkok, Chiang Mai, Chumphon, Khon Kaen and Saraburi regions. Methods: We select a random sampling method to buy 202 samples of seafood from different locations. formaldehyde detection was examined by formaldehyde test kits, manufactured by the department of medical science. Result: From the study of formaldehyde detection in water soaked in 202 seafood samples in Bangkok, Chiang Mai, Chumphon, Khon Kaen and Saraburi provinces. formaldehyde was found to be contaminated in the soaking water of 17 seafood samples, or 8.4%. Formaldehyde was found to be contaminated. From the 11 crispy squid samples, 64.7% of all samples were contaminated with formaldehyde. 4 samples of fresh squid, equivalent to 14.7 %, fresh shrimp, 1 sample, or 5.8%, and striped shellfish, 1 sample, or 5.8%. When analyzing by area, it was found that from the seafood infusion samples randomly taken from Saraburi Province, 5 of 30 samples of formaldehyde, representing 16.7%, were found in Bangkok, 9 of 85 samples, or 10.6%, and Chiang Mai, 2 of 30, or 6.7% and Chumphon Province, one sample out of 23 samples, or 4.3%. No formaldehyde was detected. Conclusion: Formaldehyde has also been found to be contaminated in seafood in many areas.