Three novel carbon-bridged citrinin dimers, dicitrinones A–C ( 1– 3), were isolated from a volcano ash-derived fungus Penicillium citrinum. Their structures were elucidated by spectroscopic methods. Dicitrinone A ( 1) and B ( 2) are two atropisomeric mixtures, which were verified by both NMR and quantum chemical calculations. Their cytotoxicity against four tumor cell lines (HL-60, MOLT-4, A-549, and BEL-7402) was evaluated by SRB and MTT methods. Dicitrinone B ( 2) arrested the HL-60 cell cycle at the G2/M phase, which was analyzed using flow cytometry.