In this work, we consider optimal discrimination among quantum states that are sent through a quantum channel. We show the conditions on a quantum channel and an ensemble of states to preserve a measurement for optimal state discrimination over the channel. In particular, we show that for an ensemble of states where the states are given with equal a priori probabilities, an optimal measurement can be preserved over any quantum channel by applying local operations and classical communication, that is, by manipulating the quantum states before and after the channel application. Examples are provided for illustration in the case of quantum communication protocols. Our results can be readily applied to optimal state discrimination over various types of noise.