Crystalline Ni₂B, Ni₃B, and Ni₄B₃ are synthesized by a single-step method using autogenous pressure from the reaction of NaBH₄ and Ni precursors. The effect of reaction temperature, pressure, time, and starting materials on the composition of synthesized products, particle morphologies, and magnetic properties is demonstrated. High yields of Ni₂B (>98%) are achieved at 2.3–3.4 MPa and ~670 °C over five hours. Crystalline Ni₃B or Ni₄B₃ form in conjunction with Ni₂B at higher temperature or higher autogenous pressure in proportions influenced by the ratios of initial reactants. For the same starting ratios of reactants, a longer reaction time or higher pressure shifts equilibria to lower yields of Ni₂B. Using this approach, yields of ~88% Ni₄B₃ (single phase orthorhombic) and ~72% Ni₃B are obtained for conditions 1.9 MPa < P_max < 4.9 MPa and 670 °C < T_max < 725 °C. Gas-solid reaction is the dominant transformation mechanism that results in formation of Ni₂B at lower temperatures than conventional solid-state methods.