Sensitivity toward humidity, response and recovery time, and reproducibility behaviors of a novel copolymer sensor, obtained by oxidative chemical polymerization of o-phenylenediamine and aniline using D-camphor-10-sulfonic acid dopant, were reported. It is a novel copolymer, and there are very few studies available concerning its synthesis and humidity sensing applications. Humidity-sensitive properties of the copolymer were tested by measuring the resistance of the fabricated films, and good humidity sensitivity was obtained. Moreover, a decrease in resistance with the increase in humidity was observed. Compared to polyaniline (PANI), the humidity sensing property of the copolymer was found to show quicker response and recovery time, higher reproducibility, and higher sensitivity. However, PANI alone exhibited relatively poor sensing property. The response time for the copolymer films at the relative humidity ranging from 11 to 97% was about 38 s, and the corresponding recovery time was about 11 s. The value of sensitivity was about 10. These results indicate excellent sensing character of the new type of copolymer humidity sensors. The obtained sensor is expected to exhibit good humidity sensing behavior in moisture-containing environment.