<p>Monitoring of ²²⁶Ra in drinking water is critical due to its radiotoxicity and potential health risks. This study presents a novel method for ²²⁶Ra determination in bottled mineral waters using a KMnO₄-modified fly ash sorbent. The sorbent, prepared by manganese dioxide surface modification, demonstrated significantly enhanced radium sorption capacity. Method validation, including linear regression and equivalence tests, confirmed that the modified fly ash sorbent and commercial MnO₂-PAN resin yield statistically equivalent results for ²²⁶Ra determination in mineral waters. Thus, it is concluded that application of the modified sorbent represents a reliable and cost-effective alternative to commercial methods. Application to 20 commercially available Slovak bottled mineral waters revealed ²²⁶Ra activity concentrations corresponding to annual committed effective doses ranging from 1 to 154 µSv/y. While the average dose (41.8 µSv/y) remained well below national and international safety limits, three samples exceeded the Slovak regulatory threshold of 100 µSv/y. These findings emphasize the need for ongoing monitoring and demonstrate the suitability of the KMnO₄-modified fly ash sorbent for routine radiological quality control of bottled mineral waters.</p>