Microwave spectroscopy of the muonium <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mn>2</mml:mn> <mml:msub> <mml:mi>S</mml:mi> <mml:mrow> <mml:mn>1</mml:mn> <mml:mo>/</mml:mo> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> <m

We report a microwave spectroscopy measurement of the muonium 2⁢<em>𝑆</em>_1/2−2⁢<em>𝑃</em>_3/2 fine-structure transition, yielding a transition frequency of 9871.0±7.0MHz, in agreement with state-of-the-art QED predictions within one standard deviation. In combination with the recent Lamb shift result, the 2⁢<em>𝑆</em>_1/2−2⁢<em>𝑃</em>_3/2 splitting is determined, improving the spectroscopic characterization of the <em>𝑛</em> = 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.