Ultrathin SnO₂ nanosheets have been successfully synthesized through a simple low temperature hydrothermal strategy and developed for formaldehyde gas detection. Morphology characterizations are confirmed by the results of FESEM, TEM and HRTEM. The ultrathin SnO₂ nanosheets are configured as high performance sensors to detect formaldehyde, and show very fast response times (<1 s), recovery times (6 s), good repeatability and selectivity at a relatively low working temperature. The high sensitivity performances are attributed to the morphology characterizations of ultrathin SnO₂ nanosheets for affording large specific surface areas and more direct conduction pathways. The growth mechanism of the ultrathin SnO₂ nanosheets and a morphology dependent sensing mechanism are proposed.