Tollmien-Schlichting waves are flowfield instabilities that can eventually evolve into turbulence, which can degrade aircraft performance. This paper presents a repetitive control approach for the attenuation of Tollmien–Schlichting (TS) waves through active flow control. Since the TS-wave behavior depends on flow conditions that may change over time, we pair the repetitive controller with an on-line period detection algorithm. This results in a repetitive controller where the internal model is time-varying. Therefore, we derive sufficient conditions under which the controller remains stable, for time-varying or incorrectly detected periods. For validation, high fidelity CFD simulations are conducted wherein TS waves are artificially generated by an upstream actuator and canceled downstream through the control scheme. The disturbance attenuation performance of the proposed solution is benchmarked against a state-of-the-art strategy, namely a Filtered-x Least Mean Squares (FxLMS) controller. The repetitive controller is shown to deliver comparable performance to FxLMS, while eliminating the need for a second upstream reference sensor (which is necessary for the FxLMS). At least 92% amplitude attenuation of both single-frequency and periodic TS waves is demonstrated in the high-fidelity simulation environment.