Rivulus giarettai differs from all other species of the subgenus Melanorivulus by possessing a unique color pattern on flank in males, in which red pigmentation is arranged to form a vermiculate pattern. It is similar to species endemic to the central Brazilian

plateau in having ventral process of angulo-articular vestigial, first epibranchial curved, and flank intense greenish blue or greenish golden, to purplish blue above anal-fin base in males; it differs from central Brazilian plateau species by a combination of reduction of black pigmentation on head and humeral region in males, 34-36 scales on the longitudinal series, 30-32 caudal-fin rays, 3-4 vomerine teeth, and caudal fin with bars in both sexes.