Diagnosis: Betta mahachaiensis is distinguishable from other congeners in the B. splendens group by the
combination of the following characters: an iridescent green to bluish-green appearance on the brown-to-black
body background; two parallel iridescent green to bluish-green vertical bars on opercle; brown-to-black opercular
membrane without red streaks or patches; dorsal, caudal, and anal fin rays brown to black contrasting with
iridescent green or bluish-green interradial membranes; dorsal fin with at least its proximal two-thirds having
transverse black bars; absence of dark transverse bars on caudal fin, small black rounded marks flanking the
internal rays on interradial membranes; pelvic fin brown to black with iridescent green/bluish-green front margin
and white tip; head length 24.9–31.2% SL; postdorsal length 17.1–25.2% SL; length of dorsal fin base 12.2–19.3%
SL; pelvic fin length 21.2–49.6%; pectoral fin length 15.4–21.3% SL; orbit diameter 22.8–29.7% SL.