In this article, we propose and implement a novel technique to locate electromagnetic interference (EMI) sources using the concept of time-reversal (TR) cavity. We show in an intuitive manner that reflections from the surfaces of a cavity can emulate an infinite number of sensors in the TR method. In order to locate EMI sources, the equipment under test (EUT) is placed in a rectangular metallic cavity, of suitable dimensions according to the considered frequency band and the EUT size, equipped with a simple monopole or dipole antenna. We demonstrate that by using only one sensor, we are able to locate EMI sources by taking advantage of the focusing properties of a TR cavity. The entropy criterion is applied to obtain the focusing time slice in which the maximum electric field determines the location of the EMI source. Both 2- and 3-D numerical simulation schemes are deployed to demonstrate the ability of the proposed technique. The validity of numerical simulations is tested against frequency domain measurements. Compared with the conventional EMI tests in anechoic chambers and scanning methods, the proposed technique represents a simpler and cost-effective test method requiring only one sensor (a monopole or dipole antenna).