This study investigates the development of room-temperature NH3 selective gas sensors based on double-shell hierarchical SnO2@polyaniline (DSPx) composites. The composites were synthesized via in situ oxidative polymerization and deposited onto a polyethylene terephthalate (PET) substrate to fabricate a flexible, electrode-free sensor. The impact of SnO2 content on the sensing performance of DSPx-based sensors for NH3 detection at room temperature was evaluated. The results showed that the 20 mol% D-SnO2@PANI (DSP20) sensor exhibited the best performance, with a response value of 37.92 to 100 ppm NH3, which is 5.1 times higher than that of a pristine PANI sensor. The DSP20 sensor also demonstrated rapid response and recovery rates at 10 ppm NH3, with response and recovery times of 182 s and 86 s, respectively. The enhanced sensing performance is attributed to the unique microstructure and the p-n heterojunction formed between the D-SnO2 surface and PANI. This study highlights the potential of DSP-based sensitive materials for highly selective NH3 detection at room temperature.This study investigates the development of room-temperature NH3 selective gas sensors based on double-shell hierarchical SnO2@polyaniline (DSPx) composites. The composites were synthesized via in situ oxidative polymerization and deposited onto a polyethylene terephthalate (PET) substrate to fabricate a flexible, electrode-free sensor. The impact of SnO2 content on the sensing performance of DSPx-based sensors for NH3 detection at room temperature was evaluated. The results showed that the 20 mol% D-SnO2@PANI (DSP20) sensor exhibited the best performance, with a response value of 37.92 to 100 ppm NH3, which is 5.1 times higher than that of a pristine PANI sensor. The DSP20 sensor also demonstrated rapid response and recovery rates at 10 ppm NH3, with response and recovery times of 182 s and 86 s, respectively. The enhanced sensing performance is attributed to the unique microstructure and the p-n heterojunction formed between the D-SnO2 surface and PANI. This study highlights the potential of DSP-based sensitive materials for highly selective NH3 detection at room temperature.