Etrumeus makiawa is described as a new species of round herring from the Hawaiian Islands. Formerly identified as E. micropus (type locality, Japan), it is distinct from that species in having modally one fewer pectoral ray and 48–51 gill rakers, compared with 44–48 for E. micropus. Japanese and Hawaiian Etrumeus compose reciprocally monophyletic mtDNA lineages (d = 4.60%) with a long period of separation (ca. 2.3 million yr). This new Hawaiian endemic is also differentiated from E. acuminatus in California and Baja California, which instead has a count of 41–45 gill rakers, a larger maximum size (to 280 mm SL, compared with 198 mm for E. makiawa), and a clearly different mtDNA sequence (d = 2.20%). The northwestern Atlantic species, E. sadina (E. teres is a synonym), has 49–54 gill rakers and is genetically differentiated from all the other species considered here (d = 15.95% to 17.58%).