The frequency-hopping technique is widely used in commercial and military communications for its superiority in anti-jamming and low probability of interception and detection capabilities. To further lower its detection probability, a frequency-hopping signal randomly changes its carrier frequency within a transmission symbol. Conventional detection methods have poor performance in the presence of fast frequency-hopping signals. To solve this problem, we propose a dirty-template-based detection scheme. Dirty Template was originally proposed to blindly estimate the time delay in an ultra-wide-band system. We propose that it can be employed in the frequency domain to detect fast frequency-hopping signal. The decision statistic derived from the cross-correlation between the template and received signals in the frequency domain is analyzed. Using this cross-correlation, a detection scheme, based on the Neyman-Pearson test is proposed. The simulation results show that the dirty-template-based scheme outperforms the autocorrelation-based scheme when the hopping period is short.