Diagnosis: Eleotris diamsoi is distinguished by second, third and fourth suborbital free neuromast rows on cheek extending ventrally past horizontal row 'd' ('2.3.4' pattern); row 'os' connected with row 'oi' at ventro-posterior margin of operculum ('closed' pattern); 16 pectoral fin rays; 63 scales in lateral series; a small body depth at anal and second dorsal fins origins, 15-18% of standard length (Ref. 123271). Eleotris diamsoi differs from other species of the Indian Ocean with a '2.3.4' pattern by several characters: it differs from E. soaresi in having 16 pectoral fin rays vs. 17-18, more scales in lateral series, 63 vs. 50-56, and in zigzag series, 14 vs. 12-13, and a smaller body depth at anal fin origin, 15-18% of standrad length vs. 20-21%; it differs from E. valadei in having 16 pectoral fin rays vs. 18, fewer scales in predorsal series in males, 36-38 vs. 45-50, fewer scales in transverse forward series, 17-23 vs. 27-30, and a smaller body depth at anal fin origin, 15-18% of standard length vs. 19-24% (Ref. 123271).

Description: The body is elongated; the body depth at anus is 15-18% of standard length and the caudal peduncle depth is 12-14% of standard length; predorsal length 44-47% of standard length and preanal length 63-66% of standard length (Ref. 123271). The head 32-33% of standard length is broad and depressed, the snout is pointed; eyes high on head; the mouth is quite large, as jaw length is 10-11% of standard length, and oblique, the posterior margin of upper jaw reaches the vertical through the two thirds of the eye; upper and lower jaws with multiple rows of small inwardly curved teeth, larger on outer row; a few caniniform teeth in anterior position on lower jaw (Ref. 123271). Dorsal fins VI-I,8; first dorsal fin spine separate from and smaller than second dorsal fin spine; spines not elongated; anal fin I,8 and directly opposite to second dorsal fin; pelvic fins separate, I,5; pectoral fins 16; caudal fin pointed with 15 branched rays (Ref. 123271).Cycloid scales on top of head, nape, cheek, operculum, pectoral fin base, prepelvic region, and abdomen; ctenoid scales covering flanks; no lateral line canals; 63 scales in lateral series, 36-38 in predorsal series, 18 in transverse back series, 17-23 in transverse forward series and 14 in zigzag series (Ref. 123271). A downward pointing spine at the posterior margin of the preoperculum is covered with skin; gill opening is moderately broad, extending to below the preoperculum; tubular anterior nares overhanging one third of upper lip, posterior nares open pits (Ref. 123271). Adults with five transverse suborbital free neuromast rows of which second, third and fourth suborbital free neuromast rows on cheek extending ventrally past horizontal row 'd' ('2.3.4' pattern); row 'os' connected with row 'oi' at ventro-posterior margin of operculum ('closed' pattern) (Ref. 123271). Urogenital papilla in females rounded, elongated and tapered in males (Ref. 123271).

Colouration: Colour in life unknown (Ref. 123271). Colour in preservation of male and female similar: head and preoperculum beige; back and flanks brownish; a big brown dot at the base of the pectoral fins; abdomen and gular region yellowish; anal and first and second dorsal fins spotted with spots aligned on rays; caudal fin brownish, with several yellow spots; pelvic and pectoral fins whitish (Ref. 123271).