The fugacity coefficients of the hydroperoxyl radical in supercritical water are estimated through molecular simulations. A potential function for the radical is first derived from ab initio self-consistent field molecular orbital calculations at the unrestricted Hartree-Fock level and from data in the literature. Molecular dynamics simulations of the hydroperoxyl radical are then performed in supercritical water and the fugacity coefficient of the radical is calculated by the free-energy perturbation method using the dynamic coupling parameter window-modification technique. The values of the fugacity coefficients at 773 K differ from unity. This methodology facilitates the incorporation of thermodynamic nonidealities in mechanism-based kinetic models for free-radical reactions in supercritical water.