Ag(0)x–Mn3O4 nanocomposite layers have been obtained under “mild” chemical conditions at room temperature by treating the surface of a solution of a mixture of AgNO3 and Mn(NO3)2 salts with ammonia gas. These layers consist of microglobules with a unique jellyfish-like morphology. The size and shape of these microglobules, as well as the density of their arrangement at the solution–air interface, are determined by the time of treatment with ammonia gas. Their comprehensive study was carried out by FESEM, TEM, HRTEM, SAED, XRD, and EDX methods and Raman and FT-IR spectroscopies. A hypothetical model is proposed to explain the formation of jellyfish-like microglobules. It was found that layers of these microglobules can be used as SERS substrates, allowing a signal amplification factor of 107 times to be achieved.