This research aimed to detect formalin in squid that was sold from a large seafood wholesale and retail market in Muang District Nakhon Ratchasima province, Thailand. The samples of squid were randomly drawn from 17 squid shops in 6 markets, 100 grams per shop and surveyed 4 times, totalling 68 samples. An innovative paper-based sensor for formalin tests in foods from Burapha University was used to test the formalin samples. The results presented that fresh squid which was sold in 6 seafood markets showed no sign of formalin. This indicated that all 6 markets selling squid in Muang district, Nakhon Ratchasima, Thailand do not smuggle in formalin to maintain freshness and prevent squid spoilage. From this study, it was concluded that there was no formaldehyde contamination in squid bought from seafood markets in the Muang district. The consumption of fresh squid which was purchased from these markets is therefore safe and free from formalin poisoning.