A new mechanism of harmonics generation is found in ultraintense laser interactions with plasmas, where radiation reaction effects dominate. The radiation reaction results in trapped electrons which oscillate in the laser field and emit high energy photons. Such emission event happens with a frequency spectrum containing double or high order components of the drive laser, inducing second and higher order harmonics emission. Although the intensity of such harmonics is too weak for practical applications, they could be used as a specific diagnostic to detect the radiation reaction effects in the upcoming experimental studies of the extremely intense laser pulse interacting with plasmas.