Monoamine oxidase (MAO) plays an important role in psychiatric and neurological disorders, such as depression and Parkinson's disease. As a result, MAO represents a key target for developing drugs to treat these conditions. The present study aimed to synthesise and discover compounds that inhibit the MAO enzymes and which may be relevant to the treatment of neurological disorders. A series of nine 2H-1,4-benzothiazin-3(4H)-ones were synthesised and evaluated as potential in vitro inhibitors of human MAO-A and MAO-B. The benzothiazinones bear structural similarity to a series of 3,4-dihydro-2(1H)-quinolinones that have been shown to be highly potent MAO-B inhibitors. The results show that the benzothiazinones inhibit both MAO isoforms but are more potent MAO-B inhibitors. The most potent inhibitors exhibit IC<sub>50</sub> values of 0.0027 (1b), 0.0082 (1c), 0.0096 (1d), and 0.0041 µM (1h) for MAO-B, and 0.714 µM (1d) for MAO-A. These benzothiazinone derivatives exhibit promising MAO inhibition activities and may be developed into clinically useful agents for the treatment of Parkinson's disease. Interestingly, related 4H-3,1-benzothiazin-4-ones have previously been investigated as dual-target-directed drugs that inhibit MAO-B and antagonize adenosine A<sub>2A</sub> receptors for the treatment of Parkinson's disease. Based on an interest in d-amino acid oxidase (DAAO) as a target for the treatment for schizophrenia, the 2H-1,4-benzothiazin-3(4H)-ones were also evaluated as in vitro inhibitors of this enzyme. Among the study compounds, only 7-hydroxy-2H-1,4-benzothiazin-3(4H)-one (IC<sub>50</sub> = 4.20 µM) was an active DAAO inhibitor. Finally, molecular docking was employed to gain insight into the binding modes and interactions of the 2H-1,4-benzothiazin-3(4H)-ones with MAO.