The straight channel optical waveguide coated with the SnO₂ nanoparticle is studied as an all-optical humidity sensor. The proposed sensor shows that the transmission loss of the waveguide increases with increasing relative humidity (RH) from 56% to 90% with very good repeatability. The sensitivity to changes in relative humidity is ~2 dB/% RH. The response time of the humidity sensor is 2.5 s, and the recovery time is 3.5 s. The response to humidity can be divided into 3 different regions, which are correlated to the degree of water adsorption in the SnO₂ nanoparticle layer. Compared with the previous all-optical humidity sensor based on SnO₂, the proposed sensor exhibits more rapid response, simpler fabrication process, and higher sensitivity. The proposed sensor has a potential application in the long distance, remote agriculture, and biological humidity sensing.