Hydroborates are an emerging class of solid electrolytes for all-solid-state batteries. Here, we investigate the impact of pressure on the crystal structure and ionic conductivity of a close-hydroborate salt consisting of Na ₂ B ₁₀ H ₁₀ and Na ₂ B ₁₂ H ₁₂ . Two Na ₂ B ₁₀ H ₁₀ :Na ₂ B ₁₂ H ₁₂ ratios were studied, 1:1 and 1:3. The anions of the as-synthesized powder with 1:1 ratio crystallize in a single face-centered cubic phase, while the anions of the powder with 1:3 ratio crystallize in a single monoclinic phase. After applying pressure to densify the powder into a pellet, a partial phase transformation into a body-centered cubic (BCC) phase is observed for both ratios. The BCC content saturates at 50 weight percent (wt%) at 500 MPa for the 1:1 ratio and at 77 wt% at 1000 MPa for the 1:3 sample. The room temperature sodium-ion conductivity follows an analogous trend. For the 1:1 ratio, it increases from 2 × 10 ^–4 Scm ⁻¹ at 10 wt% BCC content to about 1.0 × 10 ^–3 Scm ⁻¹ at 50 wt% BCC content. For the 1:3 ratio, it increases from 1.3 × 10 ^–5 Scm ⁻¹ at 11.9 wt% BCC to 8.1 × 10 ^–4 Scm ⁻¹ at 71 wt% BCC content. Our results show that pressure is a prerequisite to achieve high sodium-ion conductivity by formation of the highly conductive BCC phase.