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Abstract Despite major progress in the investigation of boron cluster anions, direct experimental study of neutral boron clusters remains a significant challenge because of the difficulty in size selection. Here we report a size‐specific study of the neutral B₉cluster using threshold photoionization with a tunable vacuum ultraviolet free electron laser. The ionization potential of B₉is measured to be 8.45±0.02 eV and it is found to have a heptagonal bipyramidD_7hstructure, quite different from the planar molecular wheel of the B₉^‐anionic cluster. Chemical bonding analyses reveal superior stability of the bipyramidal structure arising from delocalized σ and π bonding interactions within the B₇ring and between the B₇ring and the capping atoms. Photoionization of B₉breaks the single‐electron B‐B bond of the capping atoms, which undergo off‐axis distortion to enhance interactions with the B₇ring in the singlet ground state of B₉⁺. The single‐electron B‐B bond of the capping atoms appears to be crucial in stabilizing theD_7hstructure of B₉. This work opens avenues for direct size‐dependent experimental studies of a large variety of neutral boron clusters to explore the stepwise development of network structures.