  We report a new measurement of the $n=2$ Lamb shift in Muonium using
microwave spectroscopy. Our result of
$1047.2(2.3)_\textrm{stat}(1.1)_\textrm{syst}$ MHz comprises an order of
magnitude improvement upon the previous best measurement. This value matches
the theoretical calculation within one standard deviation allowing us to set
limits on CPT violation in the muonic sector, as well as on new physics coupled
to muons and electrons which could provide an explanation of the muon $g-2$
anomaly.
