In this letter, we demonstrate the existence of unstable ion temperature gradient driven Alfvén eigenmodes in tokamak plasmas, which are ideally stable with respect to magnetohydrodynamics (MHD). Conditions for the destabilization of such modes are quantitatively discussed on the basis of theoretical analyses of the mode dispersion relation, which is given in a compact analytical form. It is emphasized that instability requires both sufficiently strong thermal ion temperature gradients and that the plasma be sufficiently close to ideal MHD marginal stability.