This article presents the hitherto unknown concept of transient active power in two-terminal networks. This phenomenon occurs when current and voltage signals are not sinusoidal but quasi-sinusoidal—in the case of an approximation of transient state. The modification of Parseval’s formula and the power–immittance relations are presented in this paper. To illustrate the phenomenon, a simulation is included for several types of transient states, and their influence on transient active power waveform is shown. The article also contains a comparison of transient active power and classical active power and highlights situations where it is impossible to use classical theory but where transient active power yields measurable results. In the article, an analysis of an idealized case and measurement data obtained from a modeled voltage sag in a laboratory setup is conducted. The impact of disturbance power on the total power in the case of disturbances that may occur in the real power network is demonstrated.