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 homogenously 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 is the first example of a borosilicate compound with edge-sharing BO4 tetrahedra.