This paper presents the Cramer-Rao Bound (CRB) of time-difference-of-arrival (TDOA) estimation for frequency-hopping (FH) signals, which has still not been reported in the open literature. The CRBs are derived in closed-form for both flat fading and frequency-selective channel models. The derived bounds provide insights into the influence of the FH parameters of a FH signal on TDOA estimate accuracy over flat fading and frequency-selective channels. Simulation results are provided to verify the theoretical findings.