This paper reports the results of an investigation into the influence of precipitation and air temperature on the efficiency of pollutant removal processes and effluent pollutant concentrations in a one-stage constructed wetland with subsurface vertical flow. We studied an on-site constructed wetland system that used Phragmites australis for the treatment of domestic wastewater. The system was located in central Europe, in the south-east of Poland, in a temperate climate zone with transitional features. Physico-chemical analyses of influent and effluent wastewater, as well as measurements of precipitation and air temperature were carried out in the years 2001–2010. It was shown that the pollutant removal efficiency of the treatment plant was significantly higher in the growing season than outside the growing season (the mean efficiency is usually a few percent higher but generally this parameter is highly varied). This indicated that temperature determined the efficiency of the wastewater treatment. We found that the amount of precipitation affected the concentration of pollutants in the effluent. The more rainfall there was, the lower the content of pollutants in the effluent from the treatment plant, which demonstrated that rainwater diluted the concentrations of pollutants in the treated wastewater—thus improving the efficiency of the wastewater treatment plant.