A novel modification of hexagallium decastrontium oxide, β-Sr10Ga6O19, has been prepared from a non-stoichiometric melt of composition 4SrO:Ga2O3 in the temperature range between 1490 to 1600 °C. The crystal structure was determined at 25 °C from single crystal X-ray diffraction data (orthorhombic, space group Pbcn, a=34.3163(21) Å, b=7.8918(4) Å, c=15.9558(7) Å, V=4321.1(4) Å3, Z=8, R1=0.036, wR2=0.076 for 1839 observed reflections with I>2σ(I)). It contains highly puckered unbranched six-membered oligogroups of composition [Ga6O19]20−. The oligomers are located in 6.5 Å wide slabs oriented perpendicular to [001] and are connected via Sr-ions residing within as well as in between the slabs. Alternatively, the structure can be described as a 8×2×4 superstructure of perovskite ABO3, with 20.8% vacancies in the oxygen sublattice. From the 64 B-sites in the unit cell of β-Sr10Ga6O19, 25% are occupied by Sr- and 75% by Ga-cations, whereas the corresponding A-sites are exclusively occupied by Sr. Therefore, the relationship with perovskite can be expressed in the following crystal chemical formula: Sr(Ga3/4Sr1/4)(O19/8□5/8).