We report a microwave spectroscopy measurement of the muonium $2S_{1/2}\text{−}2P_{3/2}$ fine-structure transition, yielding a transition frequency of $9871.0\pm 7.0\mathrm{MHz}$, in agreement with state-of-the-art QED predictions within one standard deviation. In combination with the recent Lamb shift result, the $2P_{1/2}\text{−}2P_{3/2}$ splitting is determined, improving the spectroscopic characterization of the $n=2$ manifold in muonium. These results provide a stringent test of bound-state QED in a purely leptonic system and establish a path toward future searches for Lorentz violation and muon-specific new physics.

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• Exotic atoms & molecules
• Muonium