We aimed to determine the spawning period of black sea bream Acanthopagrus schlegelii in Hiroshima Bay, Japan, based on the distribution of its eggs, to assist in the resource management of this species. We developed a monoclonal antibody assay for the identification of black sea bream eggs, and appraised its specificity using immunohistochemical staining and DNA barcoding. To estimate the black sea bream spawning season in 2015 and 2016 in the major fishing ground of Hiroshima Bay, we collected 4061 eggs, attributed to pelagic black sea bream using our assay, and calculated the gonadosomatic index of 715 black sea bream adults to confirm the correspondence between egg presence and spawning season. We estimated that the spawning season of this species extended from mid-April until early July in both 2015 and 2016, and that it peaked in early May, about 20 days earlier than the peak reported for this species in the early 1980s. This temporal shift may be related to increasing seawater temperatures in Hiroshima Bay associated with climate change and ocean warming.