In recent years, hybrid manganese(II) halides (HMHs) have attracted wide attention due to their impressive optical properties, low toxicity, and facile synthetic processibility. Being effective reabsorption-free phosphors, these compounds demonstrate the potential to be used as low-cost solution-processable scintillators. However, most of the HMHs studied to date contain bulk organic cations and, as a result, are characterized by low density and low X-ray stopping power. For this reason, we studied manganese(II) bromides with compact organic cations such as formamidinium (FA+) and acetamidinium (AcA+). In particular, we synthesized four new phases, two of which are characterized by octahedral coordination of manganese ions ((FA)MnBr3 and (AcA)MnBr3) and red emission, whereas the other two have tetrahedrally coordinated Mn2+ ions ((FA)3MnBr5 and (AcA)2MnBr4) and green emission. Photoluminescence (PL) and radioluminescence measurements demonstrated high PL quantum yields and reasonable scintillation light yields of acetamidinium-based compounds. In addition, unlike most known HMH-based scintillators, the discovered materials have a relatively high density due to the small fraction of the volume occupied by organic cations, so their X-ray attenuation coefficients are comparable to the well-known oxide scintillators.