The study aims to monitor the birch pollen season in selected Polish cities: Bialystok, Bydgoszcz, Cracow, Lublin, Olsztyn, Opole, Sosnowiec, Szczecin, Warsaw, Wroclaw and Zielona Gora in 2021. Pollen concentrations were recorded using a Burkard type sampler operating in a continuous volumetric mode. The birch pollen season of 2021 started during the first and second decade of April. In south-western Poland it began about 10 days earlier (April 7th or 10th) than in north-eastern regions (April 18th or 19th). The average length of the season was 31 days. The highest daily concentrations and annual pollen grains sum (seasonal pollen integral) were the most variable season characteristics. The maximum concentrations of birch pollen were recorded between April 21st (in Wroclaw) and May 1st (in Sosnowiec and Warsaw). It ranged from 222 p/m3 in Cracow up to 1997 p/m3 in Wroclaw. SPI was the lowest in Cracow (1573) and Sosnowiec (1763) and the highest in Bydgoszcz (5474) and Wroclaw (5072). The longest exposure to high concentrations of birch pollen, lasting 16–18 days, was detected in Szczecin, Olsztyn and Bydgoszcz. The abundance of birch pollen in Poland in 2021 was not as high as in 2019 and 2020, which, combined with the protective effect of mouth and nose masks due to COVID-19 pandemic, may have resulted in less severe allergy symptoms in sensitized individuals than in previous years.