Recently low-power, small gas sensors have been strongly demanded to realize "super smart society." In particular, ammonia: NH3 sensors are expected to be key devices for breath diagnosis. However, it is difficult for NH3 sensors to obtain high selectivity against hydrogen: H2, since conventional metal-oxide gas sensors respond to any reducing gases. In this study, Co-porphyrin functionalized graphene sensors were fabricated, and selective NH3 sensing was realized by the selective metal-ligand bond of Co with NH3. The sensor successfully detected sub-ppm NH3, while it showed no response to high concentration H2. Furthermore, we investigated the effect of humidity on the Co-porphyrin functionalized graphene NH3 sensor. We demonstrated the detection of low concentration NH3 even under rapid humidity changes. We considered that the key sensing mechanism of our sensor is charge transfer to graphene by the electronic structure change of Co-porphyrin-graphene complexes due to NH3 adsorption on Co-porphyrin.