Recently, continuing our investigations on the constituents of Citrus plants after isolating many new bicoumarins and acridone-coumarin dimers, we investigated the constituents of Marsh grapefruit (C. paradisi Macf.) roots and isolated a novel type dimeric acridone alkaloid, citbismine-A, via column, centrifugal, and preparative thin-layer chromatographies using silica gel (yield 8.5×10⁻⁴%). Citbismine-A was obtained as yellow cubes (mp 335-336°C, [α]D ±0° in DMSO) with the molecular formula C35H32N2O10 (HR-MS). Its structure, consisting of a dimeric 1-hydroxy-9-acridone skeleton with an N-methyl, three methoxy groups, and a 2-hydroxyisopropyl-substituted dihydrofuran ring linking the two acridone units, was elucidated by spectroscopic methods (UV, IR, 1H/13C-NMR) and unequivocally established by X-ray crystal analysis. This is the first dimeric acridone alkaloid from the Citrus genus. Among bisacridone alkaloids, citbismine-A represents the first example classified into a third category, where the two acridone units are attached by a C-C bond between a dihydrofuran ring and aromatic rings, distinct from the previously known ether-bridged (e.g., atalanine, ataline) or C-C-bonded (e.g., glycobismine-A, -B, -C) types.