Acetaldehyde gas sensors are highly important for protecting human health. However, it is challenging to achieve rapid acetaldehyde detection at low concentrations and low operating temperatures. In this work, SnO 2 nanoparticles were synthesized by the calcination of amorphous metal−organic precursors prepared by hydrothermal treatment. The effects of different molar ratios of pure terephthalic acid (PTA)/SnCl 2 •2H 2 O on the microstructures and gas-sensing properties of the synthesized particles were thoroughly investigated. The results showed that the obtained SnO 2 particles were crystalline with sizes between 8 and 20 nm. Although the specific surface area of the SnO 2 particles increased slightly from 57.65 to 65.19 m 2 /g as the molar ratio of PTA/SnCl 2 •2H 2 O increased, prominent chemisorbed oxygen species and oxygen vacancies were only present in the SnO 2 -3 sample with a molar ratio of 1:1. Meanwhile, the sensor fabricated with SnO 2 -3 exhibited a high response value (10.69), an extremely fast response (3 s), and a good recovery (4 s) time when exposed to 40 ppm of acetaldehyde at 100 °C. More interestingly, it also exhibited a low limit of detection of 50 ppb for acetaldehyde gas, as well as good selectivity and long-term stability. A promising sensor for low-temperature and low-concentration acetaldehyde gas detection was developed, and the mechanism behind its sensing performance was thoroughly explored.