CsSc[B2P3O11(OH)(3)] was obtained by hydrothermal synthesis and represents the first alkali-metal scandium borophosphate containing boron with coordination numbers 3 and 4. The crystal structure was determined from single-crystal X-ray data: orthorhombic, space group Pnna (No. 52), a = 13.0529(15) A, b = 18.3403(17) A, c = 10.3838(12) A, Z = 8. The crystal structure contains the oligomeric unit [B2P3O11(OH)(3)](4-) in which a central borate tetrahedron is open-branched by two (OH)PO3 tetrahedra and cyclobranched by one PO4 tetrahedron and a trigonal-planar (OH)BO2 by sharing common apexes. The borophosphate oligomers together with the ScO6 octahedra are condensed to form layers. Simultaneous difference thermal analysis and thermogravimetry revealed the stepwise decomposition of the compound in the temperature range between 333 and 973 K. Partly dehydrated samples do not show any substantial rehydration behavior. The crystal structure of CsSc[B2P3O11(OH)(3)] is discussed in connection with structural motifs of related borates and borophosphates.