Seasonal changes in the distribution of an important commercial and recreational fish species, Trachurus mediterraneus (Steindachner, 1868), from the family Carangidae were investigated in the Golden Horn Estuary of Istanbul. Environmental conditions were also considered to evaluate distributional patterns in the region. Sampled fish size ranged between 7.5 and 20.1 cm in total length and 3.4 and 86.2 g in total weight during the sampling period. Adult individuals (> 12.5 cm) preferred not to be in the estuary during the summer period, while only immature adults were found with a 7.5-12.5 cm total length size range from May to August 2009. It was observed that on a regional basis this species was only found at the entrance of the Golden Horn. Environmental parameters such as surface temperature, salinity, and dissolved oxygen values did not display significant correlations with its distribution throughout the study period. However, chlorophyll a and Secchi disk values displayed significant negative correlations with the distribution of immature adults in the Golden Horn.