The time- and frequency-domain transmission line matrix (TLM) methods have been developed and shown to be effective and powerful in solving electromagnetic problems. However, their direct relationship has not been clearly described so far. In this letter, we show that if a time-domain TLM method is developed, its corresponding frequency-domain counterpart can also be obtained through the discrete time Fourier transform; in a similar manner, if a frequency-domain TLM method is developed, its time-domain counterpart can also be obtained through the inverse discrete time Fourier transform. The choice of using the frequency- or time-domain depends very much on the types of the problems to be solved and the familiarity of a user with the methods.