Betta mahachaiensis, a new species of fighting fish belonging to the Betta splendens group, is described. The fish inhabits
specifically sites with brackish water and nipa palms in the Samut Sakhon Province, Thailand. It is distinguishable from other
members of the B. splendens group in having an iridescent green/bluish-green appearance on the brown-to-black body
background. The opercular membrane has brown-to-black colour without red streaks or patches. The opercle has two parallel
iridescent green or bluish-green vertical bars. Its dorsal, caudal, and anal fin rays are brown to black, contrasting with the
iridescent green or bluish-green of the interradial membrane. Black transverse bars are present on at least the proximal two thirds
of the dorsal fin, but not on the caudal fin. The colour of the pelvic fins is brown to black with an iridescent green/
bluish-green front margin and a white tip. Females possess similar characters but are less colourful than males.