High-pressure behavior of α-Mn(BH4)2 was studied up to 29.4 GPa in diamond anvil cells using powder X-ray diffraction combined with DFT calculations and Raman spectroscopy, and two new polymorphs were discovered. The first polymorph, δ-Mn(BH4)2, forms near 1 GPa and is isostructural to the magnesium analogue δ-Mg(BH4)2. This polymorph is stable upon decompression to ambient conditions and can also be obtained by compression of α-Mn(BH4)2 in a large-volume steel press as well as by high-energy ball milling. It shows a high volumetric density of hydrogen of 125 g H2/L at ambient conditions. δ-Mn(BH4)2 was refined in the space group I41/acd with the cell parameters a = 7.85245(6), c = 12.1456(2) Å, and V = 748.91(1) Å3 at ambient conditions; it can also be described in a stable P-4n2 superstructure. Its thermal stability was studied by in situ X-ray powder diffraction and thermal analysis coupled with mass-spectroscopy. δ-Mn(BH4)2 transforms back to α-Mn(BH4)2 upon heating in the temperature range of 67–109 °C in Ar (1 bar) or H2 (100 bar) atmosphere, and a decomposition is initiated at 130 °C with the release of hydrogen and some diborane. Mn(BH4)2 undergoes a second phase transition to δ′-Mn(BH4)2 in the pressure range of 8.6–11.8 GPa. δ′-phase is not isostructural to the second high-pressure phase of Mg(BH4)2, and its structure was determined in the √2a × c supercell compared to the δ-phase and refined in the space group Fddd with a = 9.205(17), b = 9.321(10), c = 12.638(15) Å, and V = 1084(3) Å3 at 11.8 GPa. Equations of state were determined for α- and δ-Mn(BH4)2.