For predicting acoustic waves emitted from a rocket plume, the overexpansion effects on Mach 3.0 supersonic jet acoustics are investigated using an implicit large eddy simulation.AMach 2.0 supersonic free jet is computed for code validation, and the results show qualitatively good agreement with the experiments. Then, computations of three different jets (design Mach numbers 3.0, 3.5, and 4.0 with fully expanded jet Mach number 3.0) are conducted, and nondimensionalizations based on design parameters and fully expanded parameters are discussed. Acoustic far-field spectra show that nondimensionalization based on fully expanded parameters works well for the high-Mach-number overexpanded jets, as it does for the low-Mach-number underexpanded jets that were investigated in previous studies. This nondimensionalization improves the accuracy of prediction of the acoustic waves emitted from rocket plumes because one parameter, the design Mach number, can be neglected for acoustic far fields. In addition, actual overexpansion effects after nondimensionalization are discussed. A comparison of the near flowfields and acoustic fields shows that Mach wave sources move upstream because of the existence of Mach disks, which enhances shearlayer mixing. Meanwhile, the overexpanded jet, which possesses only a shock cell without Mach disks, exhibits the same Mach wave generation characteristics as an ideally expanded jet.