The monoclinic crystal structure of unquenchable phase HT-LuBO₃ was refined by the Rietveld method at 1100 °C. It crystallizes in space group C2/c and its structure is composed of the distorted [BO₄]^5– and [LuO₈]¹³⁻ polyhedra. The [BO₄]^5– tetrahedra are condensed via two vertices each to form the cyclo-[B₃O₉]^9– anions. The presence of the [BO₄]^5– units was confirmed by high-temperature Raman spectroscopy at 1100 °C. The thermal behavior of β-, π- and HT-LuBO₃ was investigated by high-temperature X-ray powder diffraction (HTXRD) and thermal analysis (TG + DSC). The crystal structure of calcite-type β-LuBO₃ expands highly anisotropically. The maximum thermal expansion is along the c axis (α_c = 11.69 ‧ 10⁻⁶ °C⁻¹ at 20 °C), i.e. perpendicular to the planes of the [BO₃]^3– triangles. A weak negative expansion occurs in the ab plane (α_a = −0.33 ‧ 10⁻⁶ °C⁻¹ at 20 °C). The structure of pseudo-wollastonite-type π-LuBO₃ expands maximally in the direction that is close to the a axis (α₁₁ = 7.68 ‧ 10⁻⁶ °C⁻¹ at 20 °C), where the flexible chains consisting of the edge-sharing [LuO₈]¹³⁻ polyhedra are located. The expansion of HT-LuBO₃ is virtually isotropic within the ab plane, where the [B₃O₉]^9– rings are located. The phase transition π- ↔ HT-LuBO₃ is reversible and isosymmetric.