Betta siamorientalissp. n., a new species of fighting fish belonging to the Betta splendensgroup, inhabiting four provinces in
the eastern Thailand, is described. The combination of slender body shape with reddish vertical bars on the black opercle and
an apparent black body distinguishes the new species from other congeners especially B. imbellis. The blacker appearance
relative to B. imbellis is due to a higher proportion of black body area relative to that occupied by blue/green iridescence of
the scales. Based on morphological similarity and fin colour pattern and vertical bar colour on the opercle, the new species
appears to be more closely related to B. imbellisand B. splendensthan other members of the B. splendensgroup.