A new approach for the detection of trimethylamine (TMA) using a recombinant cell line of Xenopus laevis melanophores was developed. The cells were genetically modified to express the mouse trace amine-associated receptor 5 (mTAAR5), a G protein-coupled receptor from the mouse olfactory epithelium, which conferred high sensitivity to TMA. Cellular responses to TMA were analyzed by two different techniques, either by absorbance measurements using a microplate reader or by cellular imaging via an inverted microscope. A focused chemical screen allowed the discovery of additional, previously unknown stimuli of mTAAR5. The developed cell-based sensor demonstrated no sensitivity to trimethylamine N-oxide (TMAO), making it suitable for a straightforward evaluation of TMA levels in fish tissue extracts. For the detection of TMA vapor, the cells were covered with agarose, which allowed for intact cell viability for at least 6 h in air. The developed gas measurement platform was able to detect TMA from 1 to 100 ppm within 35 min.