<p>This study reports the annual average inner air radon (²²²Rn) levels, as well as the variation in seasonal ²²²Rn levels in different subterranean subway tunnels (STs) in and around Kolkata, West Bengal, India using the AlphaGuard PQ2000 PRO (SaphymoGmbH/Germany) portable radon monitor. For these purposes, an air radon (²²²Rn) survey was carried out at three different inner points, namely, end point 1 (EP1), the middle point (MP), and end point 2 (EP2) of different subterranean subway tunnels (STs) in three locally prominent seasons namely monsoon23 (August to September 2023), winter24 (January to February 2024), and summer24 (April to May 2024). The year-over-year average ²²²Rn level in the STs was found to be 46.7 ± 1.3&#xa0;Bq/m³, which is higher than the world average ²²²Rn level. Results show that none of the STs have a higher average radon level than the international reference level set by the EPA or even the WHO. The background ²²²Rn level measurement at an open place nearby each measured subway entrance was also carried out for each ST. While considering seasonal variation, the ²²²Rn level was found to be highest in the summer24 compared to monsoon23 and winter24. An attempt has been made to establish any correlations between the ²²²Rn level and the captured meteorological parameters. Finally, established formulae were used to assess the annual effective dose for the security guards staying or performing their daily duties and for occasional commuters in different subterranean STs in the study area.</p>