Molybdenum trioxide is chemical compound with the formula MoO₃. This compound is produced on the largest scale of any molybdenum compound. It occurs as the rare mineral molybdite. Its chief application is as an oxidation catalyst and as a raw material for the production of molybdenum metal.

The oxidation state of molybdenum in this compound is +6.

In the gas phase, three oxygen atoms are double bonded to the central molybdenum atom. In the solid state, anhydrous MoO₃ is composed of layers of distorted MoO₆ octahedra in an orthorhombic crystal. The octahedra share edges and form chains which are cross-linked by oxygen atoms to form layers. The octahedra have one short molybdenum-oxygen bond to a non-bridging oxygen.

MoO₃ is produced industrially by roasting molybdenum disulfide, the chief ore of molybdenum:

The laboratory synthesis entails the acidification of aqueous solutions of sodium molybdate with perchloric acid:

The dihydrate loses water readily to give the monohydrate. Both are bright yellow in color.

Molybdenum trioxide dissolves slightly in water to give "molybdic acid". In base, it dissolves to afford the molybdate anion.

Molybdenum trioxide is used to manufacture molybdenum metal, which serves as an additive to steel and corrosion-resistant alloys. The relevant conversion entails treatment of MoO₃ with hydrogen at elevated temperatures:

It is also a component of the co-catalyst used in the industrial production of acrylonitrile by the oxidation of propene and ammonia.

Because of its layered structure and the ease of the Mo(VI)/Mo(V) coupling, MoO₃ is of interest in electrochemical devices and displays. Molybdenum trioxide has also been suggested as a potential anti-microbial agent, e.g., in polymers. In contact with water, it forms H⁺ ions that can kill bacteria effectively. However, the issue of keeping the catalyst clean in an environment that would exploit such antimicrobial properties has not been explored.

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