We report the results of our structural reinvestigation and a more plausible solution for an acentric barium borate, BaB8O13, first reported decades ago by Krogh-Moe and Ihara in 1969. Its crystals exhibit pseudomerohedral twinning with two orthorhombic pseudotetragonal domains rotated by 90° around the common b-axes, which results in a particularly complex diffraction pattern. According to the new structure solution, BaB8O13 is the most complex octaborate (IG,total = 964.860 bits/unit cell). Its crystal structure is based on a boron-oxygen framework formed by one [B5O10]-pentaborate, three [B3O7]-triborate, and one [B2O5]-diborate groups and can be described as 16:∞3[(5:4Δ+ □) + 3(3:2Δ+ □) + (2:2Δ)]. The synthesized crystals of BaB8O13 were characterized by single-crystal and powder X-ray diffraction, thermal analysis, second harmonic generation (SHG) measurements, vibrational spectroscopy, and first-principles calculations. The cutoff edge starts at 186 nm. The compound exhibits negative thermal expansion along [001] at 100-400 °C range: αa = 6.9, αb = 11.0, αc = -0.5 × 10-6 °C-1, αV = 17.0 × 10-6 °C-1. Upon heating, the c/2a ratio approaches unity, coming close to the proposed high-temperature tetragonal "aristotype"structure. A crystal-chemical comparison as well as topological and complexity analyses of octaborates is given.