Background: The Bay Bolete (Imleria badia Vizzini) is a prized edible wild mushroom. The fungus is unfortunately, also prone to mycelial uptake of anthropogenic radiocaesium (^134/137Cs) from contaminated substrates, which accumulates in the fruiting bodies. Objective: To investigate, in light of historical atmospheric releases (including from Chernobyl) of radionuclides, the temporal and spatial distribution of radiocaesium and radiopotassium (⁴⁰K) activities in this species from eleven regions of Poland over the period 1995 to 2023. Literature data on this species over the same period were reviewed to supplement and support the findings. Material and Methods: Imleria badia samples collected across eleven regions of Poland affected by radioactive fallout were determined for ¹³⁷Cs and ⁴⁰K activity by gamma spectrometry using a coaxial high purity germanium detector. The activity measurements were verified by using a reference solution of gamma emitting isotopes (BW/Z-62/27/07) for calibration. Results: ¹³⁷Cs activity was detected in all but one sample with values ranging from < 6.4 to 11000 Bq∙kg^-1 dw. ¹³⁴Cs activity was not detected – in line with its shorter half-life. ⁴⁰K activity remained relatively stable despite the variable amounts of ¹³⁷Cs activity, confirming the validity of the measurements. Conclusions: The high values of BCF calculated (up to 134) confirm the propensity of I. badia to accumulate harmful ¹³⁷Cs activity. In some cases, the measured activity was above the regulatory tolerance limits set for human dietary exposure. Although individual regional sampling sites varied in number, the mean data for the regions indicated that the North Eastern part of Poland was most affected by ¹³⁷Cs contamination. The temporal trend shows a decline in mean ¹³⁷Cs activity from 5617 Bq∙kg^-1 dw in the 1990s to 1095 Bq∙kg^-1 dw in 2020.