Pentacarbon dioxide, officially penta-1,2,3,4-tetraene-1,5-dione, is an oxide of carbon (an oxocarbon) with formula C₅O₂ or O=C=C=C=C=C=O.

The compound is the third member of the series C_nO₂ with odd n, the first two members being carbon dioxide (CO₂) and carbon suboxide (C₃O₂). The compound was described in 1988 by G. Maier and others, who obtained it by pyrolysis of 1,3,5-cyclohexanetrione (phloroglucin, the tautomeric form of phloroglucinol). It has also been obtained by flash vapor pyrolysis of 2,4,6-tris(diazo)cyclohexane-1,3,5-trione (C₆N₆O₃). It is stable at room temperature in solution. The pure compound is stable up to −96 °C, when it polymerizes.