ZnO-Al2O3 nanocomposite was synthesized and developed as a high performance sensitive and selective layer for surface acoustic wave (SAW) sensor, aiming for in-situ detection of H2S gas in ppb level operated at room temperature. ZnO-Al2O3 nanocomposite, synthesized though a sol-gel method, was spin-coated onto a quartz based SAW resonator. This composite layer inherits the mesoporous structure of the Al2O3 layer and good affinity to H2S gas molecules of the ZnO layer, and thus can selectively adsorb and react with H2S gas molecules to form ZnS compounds on its surface. This reaction leads to significant decreases of both pore sizes and total pore volume of the layer, an increase of layer’s elastic modulus, thus causing a large positive shift of the frequency responses of the SAW sensor. The sensor operated at room temperature shows a frequency response of ∼500 Hz to 10 ppb H2S, with an excellent selectivity and good recovery property.