Atomic-level structure characterization is central to the science and engineering of materials. However, the crystal structure determination of magic-sized nanoclusters (MSCs), typical nuclei of the semiconductor nanocrystals, is impeded by challenges in obtaining phase-pure MSCs. Herein, we report on the synthesis and X-ray crystal structure of ∼0.9-nanometer-sized 27-atom semiconductor MSC, Cd14Se13. Its structure has a central Se atom encapsulated by a Cd14Se12 cage with an adamantane-like CdSe arrangement. Two chloride ions released in situ from the dichloromethane solvent stabilize and linearly self-assemble the clusters. The formation and growth pathways of Cd14Se13 clusters provide vital insights into the size-structure-property relationships in MSCs. Furthermore, potential sites for magnetic dopants are identified in Cd14Se13 MSC, enabling research on atomically precise dilute magnetic semiconductors.