A highly selective formaldehyde (hcho) gas sensor using a tin oxide nanoparticles−reduced graphene oxide (rgo−sno2) composite has been fabricated and investigated for room-temperature sensing. the rgo−sno2 composite is synthesized by cost-effective wet chemical method. the fabricated sensor exhibits remarkable sensing performance, including a higher response, a low detection limit of ≈33 ppb (theoretical), good selectivity, and long-term stability. furthermore, the effects of humidity level and calcination temperature have also been investigated. the response and recovery times for 10 ppm hcho vapor are found to be 35 and 10 s, respectively. the promising results in the lab-scale hcho detection with pre-treated fish opens up avenues for the non-invasive detection of hcho in food adulteration. the density functional theory study is in good agreement with the experimental results. the electronic charge density differences of the rgo−sno2 composite in the proximity of hcho show significant variation. in addition, the sensing mechanism of the rgo−sno2 composite toward hcho has also been discussed.