The high-pressure behavior of the borosilicate reedmergnerite NaBSi₃O₈ has been studied using in situ single-crystal X-ray diffraction and Raman spectroscopy up to 35 GPa. The crystal structure of NaBSi₃O₈ contracts homogeneously upon compression up to 12 GPa, while at higher pressures it undergoes two phase transitions. Above 16 GPa the unit-cell volume is doubled, whereas the coordination numbers of all cations and the structural topology are preserved. Above 25 GPa the crystal structure of NaBSi₃O₈ contains extremely rare dimers of edge-sharing BO₄ tetrahedra and earlier unknown Si₂O₅ groups consisting of edge-sharing SiO₅ square pyramids. The structural model was corroborated by DFT calculations. This HP modification results in the first example of a borosilicate compound with edge-sharing BO₄ tetrahedra.