Comparative notes: Betta mahachaiensis is distinguishable from other congeners in B. splendens group by
having the opercle with two parallel iridescent green to bluish-green vertical bars (vs. parallel red vertical bars in B.
splendens and green iridescent scales in B. smaragdina and B. stiktos) (see Fig. 4); brown-to-black opercular
membrane without red streaks or patches (vs. presence of red streaks in B. splendens, B. imbellis, B. smaragdina,
and B. stiktos); caudal fin rays brown to black, contrasting with iridescent green to bluish-green interradial
membranes (vs. presence of distal red crescent with a thin black edge in B. splendens and B. imbellis); absence of
dark transverse bars on caudal fin (vs. presence in B. stiktos); anal fin rays brown to black with iridescent green to
bluish-green on interradial membranes (vs. distal half of posterior anal fin rays red with a small red patch at distal
tip in B. splendens and B. imbellis); pelvic fin brown to black with iridescent green to bluish-green front margin
and white tip (vs. black and red from proximal pelvic fin base to the white tip in B. splendens and B. imbellis);
smaller head length than B. splendens and B. smaragdina (24.9–31.2% SL vs. 26.6–33.1); smaller postdorsal
length than B. splendens and B. smaragdina (17.1–25.2% SL vs. 19.1–27.6); smaller length of dorsal fin base than
B. smaragdina (12.2–19.3% SL vs. 14.6–19.9); smaller pelvic fin length than B. smaragdina (21.2–49.6% SL vs.
28.3–25.2); smaller pectoral fin length than B. smaragdina (15.4–21.3% SL vs. 15.3–24.4); smaller orbit diameter than B. smaragdina (22.8–29.7% SL vs. 23.0–31.4).