The antidepressant effects of Morinda citrifolia (noni) fruit extracts were investigated in vitro with the Monoamine Oxidase (MAO) A and B bioassays. The ethyl acetate extract of freeze-dried noni fruit powder inhibited MAO-A and B enzymes 78 and 49%, respectively. A phytochemical study of the most active extract led to the isolation of nine compounds. Among these, three principles including two flavonoids, kaempferol and quercetin, as well as one lignan, (+)-3,4,3',4'-tetrahydro-9,7'α-epoxylignan-7α,9'-lactone, were found to be potent MAO-A and B inhibitors. These findings indicate that noni fruit is a natural MAO-A and MAO-B inhibitor, involving a synergistic effect from multiple active components.