The study aims to determine the reproductive biology and 50% maturity length of red mullet fish (M. barbatus) by taking monthly samples in the Black Sea between October 2017 and September 2018. It was determined that the difference between the sexes of the samples (female: 936, male: 454) was significant and methods used to determine the reproductive period (GSI, macroscopic analysis of gonads, microscopic analysis of gonads) matched the monthly analysis results. The reproduction period of red mullet was between April and August in the Black Sea, but spawning occurred between May and August according to the histological method. Considering the oocyte structures and oocyte diameter distributions in the histological sections, it was observed that the red mullet in the Black Sea was a multiple spawner and had an indeterminable fecundity. Mean FB hydrated oocytes and mean FR hydrated eggs of red mullet during the spawning period were calculated as 4813.0±5324.0 and 124.6±124.1 g-1, respectively. The maturity sizes of females and males were 12,40 cm and 11,29 cm, respectively. As a result, for the sustainability of red mullet stocks in the Black Sea, a new management plan should be established, considering spawning periods, reproductive strategy, and maturity sizes.