Constructed wetlands (CWs) are becoming a popular solution for the treatment of rainwater discharged from hardened surfaces, because their construction and maintenance does not require large expenditures. The research on the effectiveness of treating runoff from the dairy farm yard was carried out using constructed wetland. The studied wetland was built in the form of two connected reservoirs with an area of 30 m2. In the first deep reservoir, sedimentation of suspended matter occurred, while in the second shallow reservoir, six species of aquatic plants were planted in order to assimilate nitrogen and phosphorus compounds. In the years 2014–2018, the water samples from the inflow and outflow of the constructed wetland were collected. The pH and electrical conductivity values, as well as nitrate nitrogen, ammonium nitrogen, phosphates phosphorus and chlorides concentrations were determined with electrochemical and photometric methods. The obtained results were statistically analyzed using the Statistica v. 7 software. In the majority of the runoff samples from the farmyard, abnormal concentration of ammonium nitrogen (90% of samples) and phosphate phosphorus (100% of samples) occurred in relation to the requirements for surface waters. The values of ammonium nitrogen concentration exceeded the acceptable norms for domestic sewage discharged into waters in 31% of samples and in the case of phosphate phosphorus – in 67% of samples. The concentration of chlorides exceeded the limit values for surface water in 36% of samples. The average concentration of nitrate nitrogen did not exceed these standards. There was a high correlation between the concentration of ammonium nitrogen and the concentration of phosphate phosphorus and chlorides. The efficiency of runoff treatment from the dairy cattle farm yard was very high in the initial period of research, reaching 85.4% for ammonium nitrogen and 68.3% for phosphate phosphorus, but showed a downward trend over time for the entire period.