Single crystals of new boron-aluminum-silicon nitrides, Sr3BAl5Si9N20 and Sr2.91Eu0.09BAl5Si9N20, were synthesized by heating binary nitride mixtures at 2030 °C under a N2 pressure of 0.85 MPa. The X-ray diffraction spots from single crystals of these two compounds were indexed with the trigonal cell parameters a = 22.7406(8) Å, c = 5.7066(2) Å and a = 22.7439(8) Å, c = 5.7050(2) Å, respectively, and the crystal structures were determined to have the space group P3c1, with B atoms situated at planar 3-fold-coordinated N sites. A three-dimensional framework structure is constructed for these materials based on the sharing of N atoms of Al/Si-N4 tetrahedra and B-N3 triangles. In this framework, Sr/Eu atoms are located at three sites, surrounded by 10 N atoms. Single crystals of Sr2.91Eu0.09BAl5Si9N20 emitted yellow light with a peak wavelength of 565 nm and a full width at half-maximum of 106 nm under 450 nm light irradiation. The emission intensity of these crystals at 200 °C was found to be 12% of the intensity at 25 °C.