This paper reports a solvent-regulated, surfactant-free hydrothermal process at 180 °C for 24 h to synthesize tungsten oxide (WO3) nanocrystals with various morphologies (urchin-like, rod-like and plate-like WO3) using WCl6 as the W source and just changing the solvents (from ethanol, ethanol/water (1:1 in v/v) to water); then those WO3 nanocrystals are used as the active materials to fabricate gas sensors. The results of XRD, SEM, TEM and N2 adsorption-desorption isotherms indicate that the sample obtained in ethanol takes on a urchin-like morphology (U-WO3) with apparent diameters of 1-2 μm derived by radially aggregating WO3 nanorods (∼5 nm in diameter), different from the sample obtained in the mixture of ethanol/water (1:1 in v/v), showing a rod-like morphology (R-WO3 with diameters of ∼5 nm), and the product synthesized in water, showing a plate-like shape (P-WO3) with dimensions of (100–200 nm) × (100–200 nm) × (20–30 nm). Common volatile organic compounds (acetone, methanol, methanal, ethanol and benzene) and some gases (H2, CO and H2S) are used as the target substances to evaluate the gas-sensing performance of the WO3 sensors obtained. The U-WO3 sensor is highly sensitive to acetone vapors, whereas the R-WO3 and P-WO3 sensors are highly sensitive to H2S. The morphology-dependent gas-sensing performance can be attributed to the difference in microstructures and crystalline states of the WO3 nanocrystals obtained in different solvents.