The direct synthesis of tin dioxide (SnO₂) nanowire arrays on a glass substrate by using an ultrasonic spray pyrolysis method combined with sintering is demonstrated. The products obtained are characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM) and high-resolution TEM. The results show that the SnO₂ nanowire arrays consist of single crystalline nanowires, each with a diameter of 50–70 nm and a length of 5–7 μm. There are two different nanowire growth directions because of the oxygen defect growth. The mechanism of the formation and growth of SnO₂ nanowire arrays was investigated. A platform gas sensor based on these arrays was fabricated. The sensor exhibits better sensitivity to and selectivity for NO₂ than do SnO₂ nanoparticles. The gas sensing mechanism is also discussed.