The 2p → 1s transition energy in muonic 9Be was measured using a metallic magnetic calorimeter, resulting in E(2p→1s) = 33 391.48(34) eV. The result is 30 times more precise than the previous best measurement and enables the extraction of the corresponding nuclear charge radius rc(9Be) = 2.5515(52) fm. It is 2.3 times more precise than the commonly used value based on electron scattering and differs from it by 2.5 times the combined uncertainties. This measurement represents the first determination of a nuclear charge radius using muonic x-ray spectroscopy with microcalorimeters.