The structure and thermal decomposition of Y(BH4)3 is studied by in situ synchrotron radiation powder X-ray diffraction (SR-PXD), 11B MAS NMR spectroscopy, and thermal analysis (thermogravimetric analysis/differential scanning calorimetry). The samples were prepared via a metathesis reaction between LiBH4 and YCl3 in different molar ratios mediated by ball milling. A new high temperature polymorph of Y(BH4)3, denoted β-Y(BH4)3, is discovered besides the Y(BH4)3 polymorph previously reported, denoted R-Y(BH4)3. β-Y(BH4)3 has a cubic crystal structure and crystallizes with the space group symmetry Pm3m and a bisected a-axis, a = 5.4547(8) A, as compared to α-Y(BH4)3, a = 10.7445(4)A (Pa3). β-Y(BH4)3 crystallizes with a regular ReO3-type structure, hence the Y3+ cations form cubes with BH4- anions located on the edges. This arrangement is a regular variant of the distorted Y3+ cube observed in α-Y(BH4)3, which is similar to the high pressure phase of ReO3. The new phase, β-Y(BH4)3 is formed in small amounts during ball milling; however, larger amounts are formed under moderate hydrogen pressure via a phase transition from α to β-Y(BH4)3, at∼180 °C. Upon further heating, β-Y(BH4)3 decomposes at∼190 °C to YH3, which transforms to YH2 at 270 °C. An unidentified compound is observed in the temperature range 215-280 °C, which may be a new Y-B-H containing decomposition product. The final decomposition product is YB4. These results show that boron remains in the solid phase when Y(BH4)3 decomposes in a hydrogen atmosphere and that Y(BH4)3 may store hydrogen reversibly