Atmospheric composition is an important indicator of habitability and life. The presence or absence of a large exomoon around an Earth‐size exoplanet could have important consequences for planet climate stability. Thus the detection of exomoons and retrieval of information regarding atmospheric composition of Earth‐size exoplanets are important goals of future exoplanet observations. Here a data analysis method is developed to achieve both goals simultaneously, based on reflection spectra of exoplanet‐exomoon systems. We show that the existence of exomoons, the size of exomoons, and the concentrations of some atomic and molecular species in the atmospheres of their hosting Earth‐like exoplanets can be retrieved with high levels of reliability. In addition, the method can provide well‐constrained fractions of basic surface types on the targets because of the characteristic spectral features of atmospheric species and surface types in the analyzed spectral range.