A novel SNR estimator is presented for efficient estimation of SNR in wireless OFDM systems. Unlike the conventional SNR estimators, the proposed estimator calculates an SNR estimate in the time domain, which alleviates the usage of FFT. In addition, the SNR estimator effectively exploits a periodic feature of the OFDM preamble for accurate SNR estimation. Furthermore, the SNR estimator shows robustness to timing-offset. The simulation results indicate that the presented estimator is superior to the conventional SNR estimators in terms of NMSE in the case of imperfect synchronization under high frequency-selectivity channel.