[en] Highlights: • B_n⁻ monoanions have been systematically investigated up to n = 30. However, B₂₆⁻ has remained elusive in this size range. • Here we present a joint photoelectron spectroscopy and first-principles study on the structures and bonding of this seemingly enigmatic cluster. • Extensive global minimum searches and high-level calculations reveal that isomer I dominates the experimental spectrum and represents the smallest 2D boron cluster with a hexagonal vacancy. • Isomer III is found to contribute to the measured PE spectrum as a minor species. • Chemical bonding analyses show that isomer I can be viewed as an all-boron analog of the polycyclic aromatic hydrocarbon C₁₇H₁₁⁺. Anionic boron clusters have been systematically investigated both experimentally and theoretically up to 30 atoms and have all been proved to be planar or quasi-planar (2D) in their global minima. However, the B₂₆⁻ cluster has remained elusive in this size range up to now, because of its complicated potential landscape. Here we present a joint photoelectron spectroscopy (PES) and first-principles study on the structures and bonding of this seemingly enigmatic cluster. Extensive global minimum searches, followed by high-level calculations and Gibbs free energy corrections, reveal that at least three 2D isomers, I (C₁, ²A), II (C₁, ²A), and III (C₁, ²A), could contribute to the observed PE spectrum for the B₂₆⁻ cluster. Isomer I, which has the lowest free energy at finite temperatures, is found to dominate the experimental spectrum and represents the smallest 2D boron cluster with a hexagonal vacancy. Distinct spectral features are observed for isomer III, which has a pentagonal hole and is found to contribute to the measured PE spectrum as a minor species. Isomer II with a close-packed triangular 2D structure, which is the global minimum at 0 K, may also contribute to the observed spectrum as a minor species. Chemical bonding analyses show that the principal isomer I can be viewed as an all-boron analog of the polycyclic aromatic hydrocarbon C₁₇H₁₁⁺ in terms of the π bonds.