Objective: Evaluate the test–retest reliability of quadriceps corticospinal excitability and intracortical excitability using transcranial magnetic stimulation (TMS). Design: A test–retest observational cohort study. Methods: Twelve healthy adults attended two laboratory sessions, seated with their knee at 90 degrees and fitted with electrodes on the rectus femoris (RF), vastus lateralis (VL) and vastus medialis (VM). TMS was used to assess the active motor threshold (AMT), motor evoked potential (MEP) amplitude, short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF). Individual results were calculated as the mean and median of the 10 trials of each measure for MEP, SICI and ICF. Intraclass correlation coefficients were calculated. Results: All muscles showed good or excellent reliability for the mean and median measures of the MEP amplitude (ICC ≥ 0.820) as well as the AMT of the RF (ICC = 0.991). SICI showed good reliability in the mean and median measures of the RF and the mean measure of the VL (ICC ≥ 0.809), moderate reliability in both measures of the VM (ICC ≥ 0.655) and was not significant for the median measure of the VL (ICC = 0.513). ICF showed excellent reliability in the mean measure of each muscle and median measure of the RF (ICC ≥ 0.906), with good reliability in the median measure of the VL (ICC = 0.888) and moderate reliability in the median measure of the VM (ICC = 0.719). Conclusion: The mean of an individual’s quadriceps corticospinal excitability and intracortical excitability have good or excellent reliability for every TMS measure, in every muscle (except SICI of the VM). With the previous reliability of TMS measures mostly investigating the upper limbs, these results offer important context for neurophysiological research in the quadriceps.