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In this study, we developed a new chemi-resistive, flexible and selective ammonia (NH₃) gas sensor. The sensor was prepared by depositing thin film of polyaniline-cobalt ferrite (PAni-CoFe₂O₄) nanocomposite on flexible polyethylene terephthalate (PET) through an in situ chemical oxidative polymerization method. The prepared PAni-CoFe₂O₄ nanocomposite and flexible PET-PAni-CoFe₂O₄ sensor were evaluated for their thermal stability, surface morphology and materials composition. The response to NH₃ gas of the developed sensor was examined thoroughly in the range of 1–50 ppm at room temperature. The sensor with 50 wt% CoFe₂O₄ NPs content showed an optimum selectivity to NH₃ molecules, with a 118.3% response towards 50 ppm in 24.3 s response time. Furthermore, the sensor showed good reproducibility, ultra-low detection limit (25 ppb) and excellent flexibility. In addition, the relative humidity effect on the sensor performance was investigated. Consequently, the flexible PET-PAni-CoFe₂O₄ sensor is a promising candidate for trace-level on-site sensing of NH₃ in wearable electronic or portable devices.