Exposure to poisonous and hazardous gases such as NH₃, NO₂, and CO poses significant health risks. High-performance flexible gas sensors are essential for wearable real-time hazardous gas monitoring. Polyaniline (PANI) materials for gas sensing have advantages over conventional sensing materials like low working temperature, simple manufacturing, low cost, and high flexibility, which is particularly well suited for integrating into the developing Internet of Things to achieve real-time health monitoring or human–computer interaction. However, the PANI gas sensors still need to be improved with respect to their sensitivity, gas selectivity, or other aspects for practical applications. This article reviews three aspects including the PANI gas sensing mechanism, the nanostructure of PANI materials, and mixtures of PANI with other materials as flexible sensors. Nanostructures and nanomorphologies improved the sensor response due to the higher surface-to-volume ratio and abundant adsorption sites. Many innovative approaches to manufacturing composites allow the formation of heterojunctions at the interface to achieve better performance than chemiresistive PANI gas sensors. The synthesis protocols, structure engineering, underlying mechanisms, challenges, and future prospects of PANI sensors are also comprehensively reviewed.