Reward-learning theory views habits as stimulus-response links formed through extended reward training. Accordingly, animal research has shown that actions that are initially goal-directed can become habitual after operant overtraining. However, a similar demonstration is absent in human research, which poses a serious problem for translational models of behavior. We propose that response-time (RT) switch cost after operant training can be used as a new, reliable marker for the operation of the habit system in humans. Using a new method, we show that RT switch cost demonstrates the properties that would be expected of a habitual behavior: (a) it increases with overtraining, (b) it increases when rewards are larger, and (c) it increases when time pressure is added to the task, thereby hindering the competing goal-directed system. These results offer a promising new pathway for studying the operation of the habit system in humans. (PsycInfo Database Record (c) 2020 APA, all rights reserved).