A facile approach for the fabrication of monolayer SnO2 nanonet is presented using polymer colloid monolayer nanofilms from oilwater interface self-assembly as sacrificial templates. The hole size of the nanonets can be adjusted easily by the mean diameter of polymer colloidal spheres. This method can be extended to the fabrication of a series of monolayer nanonets of semiconducting oxides such as TiO2, ZnO, and CeO2. Furthermore, the first photoresponse nanodevice based on monolayer SnO2 nanonet is fabricated. This device presents ultrahigh photocurrent and sensitivity, excellent stability, and reproducibility.