Thiazine /ˈθaɪəziːn/ is an organic compound containing a ring of four carbon, one nitrogen and one sulfur atom.

There are three isomers of thiazine, 1,2-thiazine, 1,3-thiazine, and 1,4-thiazine, which differ by the arrangement of the nitrogen and sulfur atoms in the ring.

Derivatives of thiazine, often referred to as thiazines, are used for dyes, tranquilizers and insecticides.

1,4-thiazine can be prepared from the corresponding dione using aluminium powder at high temperature.

[1] Three tautomers of 1,4-thiazine exist as above.