To overcome the multipath interference in locating transient electromagnetic (EM) radiation sources in an indoor environment, we propose a criterion that calculates the correlation between back-propagated signals from observation points, to be used in EM time reversal (EMTR) algorithms. The method introduced in this article has three main advantages with respect to classical methods that use full-wave simulations and other criteria, such as maximum field strength. First, compared with full-wave techniques in the back-propagation phase, the proposed correlation-based method utilizes approximated transfer functions from the ray-tracing technique, which can improve the computation efficiency. Second, an inverted-loss model is used for the back-propagation, which could reduce the localization error caused by multipath effects due to signal attenuation and time delay. Third, the proposed correlation criterion has weak correlation with the source characteristics, which makes it applicable to the localization in indoor reflective environments with only two observation points. Several numerical simulations are carried out to assess the performance of the proposed method. The results indicate that the proposed correlation-based EMTR technique is able to locate radiation sources accurately and efficiently in indoor reflective environments.