In enzymology, an alkanesulfonate monooxygenase (EC 1.14.14.5) is an enzyme that catalyzes the chemical reaction

an alkanesulfonate (R-CH₂-SO₃H) + FMNH₂ + O₂ ${\displaystyle \rightleftharpoons }$ an aldehyde (R-CHO) + FMN + sulfite + H₂O

The 3 substrates of this enzyme are alkanesulfonate (R-CH₂-SO₃H), FMNH₂, and O₂, whereas its 4 products are aldehyde, FMN, sulfite, and H₂O.

This enzyme belongs to the family of oxidoreductases, specifically those acting on paired donors, with O₂ as oxidant and incorporation or reduction of oxygen. The oxygen incorporated need not be derived from O₂ with reduced flavin or flavoprotein as one donor, and incorporation of one atom of oxygen into the other donor. The systematic name of this enzyme class is alkanesulfonate, reduced-FMN:oxygen oxidoreductase. Other names in common use include SsuD, and sulfate starvation-induced protein 6.