Two closely related, commercially important threadfin breams, Nemipterus furcosus and N. peronii, are an essential coastal fishery resource in Okinawa Prefecture. The age, growth, reproductive cycle, and stomach contents from 124 N. furcosus and 37 N. peronii, obtained from November 2011 to December 2015, were examined, age being assessed from sectioned otoliths and gonadal histology. Nemipterus furcosus and N. peronii were the most abundant threadfin breams in Kin Bay and Nakagusuku Bay, Okinawa Island, areas including many coastal tidal flats, where the two species comprised 98.1% of the total number of Nemipterus individuals examined. Overall sex ratios of both species were significantly sex-biased, the apparent lack of transitional gonads implying functional gonochorism. The spawning seasons of both species were estimated as occurring between spring and fall, no immature fishes having been obtained. Age validation using edge-type analyses implied that opaque zones were formed once per year, being valid annual growth increments. Although no intersex differences in maximum length, growth equation, and age range were observed in N. furcosus, N. peronii females were larger and older than males. The greatest ages observed were 4.3 and 7.0 years for N. furcosus and N. peronii, respectively. Both species fed predominantly on crabs, which primarily occupied the inner bays. Over the previous 27 years, the catch per unit effort of Nemipterus has declined in the highly altered environments of Kin and Nakagusuku Bays, suggesting that the decline in the populations of these species at Okinawa Island may be due to coastal fishery practices, environmental decline, and the degradation of suitable habitats. The biological implications for conservation are discussed.