This study investigates the impact of extreme weather conditions on the performance of a wastewater treatment plant (WWTP) in Como, Italy, in terms of influent (inf) and effluent (eff) quality parameters. During winter (October–December), the average temperature was 8.76 °C ± 11.43 °C, with 7.01 mm rainfall, while summer (May–September) averaged 23.24 °C ± 6.2 °C with 5.2 mm rainfall. Despite seasonal variations, the pH levels remained consistent. Phosphorus removal by the WWTP was efficient, with the winter influent averaging 4.16 ± 5.53 mg/L, the winter effluent averaging 0.33 ± 1.06 mg/L, the summer influent averaging 3.53 ± 2.9 mg/L, and the summer effluent averaging 0.31 ± 0.75 mg/L. The COD and BOD5 levels showed seasonal trends, with a higher winter-influent COD (450.43 ± 560.56 mg/L) than in summer (410.96 ± 302 mg/L). These higher winter values of effluent may be due to lower biological activity at cooler temperatures, affecting the efficiency of organic matter breakdown and treatment. The winter influent BOD averaged 249.57 ± 220.42 mg/L, with the winter effluent being 2.95 ± 2.04 mg/L, while the summer influent BOD was 214.44 ± 345.5 mg/L and the summer effluent was 3.01 ± 7.5 mg/L. The TSSs and Total-N showed similar seasonal patterns, with there being slight decreases in the TSSs removal efficiency during warmer months. Although microplastic pollution was not directly analyzed in this study, wastewater treatment plants play a crucial role in mitigating microplastic contamination. Despite rainfall influencing the phosphorus and organic load concentrations, the studied plant maintained over 90% pollutant removal efficiency, demonstrating resilience and compliance with regulatory standards. The WWTP’s consistent COD and BOD5 reductions highlight its robust performance amid climate variations