A remarkable life cycle of the protandrous blackhead seabream (Acanthopagrus schlegelii), initiating as a male during the first two years and then naturally transforming to a female since the third year, makes this fish a valuable model for studying molecular mechanisms of sex change. Here, we constructed a gap-free telomere-to-telomere (T2T) genome assembly for a male blackhead seabream, by integration of PacBio HiFi, Ultra-long ONT and Hi-C sequencing techniques. With 97.87% of the entire sequences anchored into 24 chromosomes, this haplotypic genome assembly spans 714.98 Mb. In terms of correctness (quality value QV: 52.95) and completeness (BUSCO score: 99.9%), this chromosome-scale assembly is indeed of high quality. It has been annotated with 24,581 protein-coding genes, and predicted with low percentage (30.95%) of repetitive sequences. As the first reference T2T-level genome assembly of various protandrous fishes, it provides a valuable genetic resource for expansion of fish genomics database. It will also allow for in-depth genomic comparisons among diverse hermaphrodite vertebrates, as well as offer fundamental genome data to support extensive research on blackhead seabream.