The life history of the Japanese sternaspid polychaete Sternaspis costata von Marenzeller, 1879 was investigated in the inner part of Ariake Bay from May 2010 to May 2011, with additional sampling in July and September 2011. All the worms were measured by the width of the ventral shield (WS) as an indicator of body size, and their coelomic contents were also examined. Ovigerous females occurred throughout the year, except during October and March, with the highest ovigerous rate (number of ovigerous females/number of adults with a WS larger than that of the smallest ovigerous female, i.e. 1.8 mm) noted in September (43.8% in 2010, 34.7% in 2011). The ovigerous rate was less than 10% in the other months, except in April 2011 (30%). Although the coelomic oocytes ranged from 90 to 160 μm in diameter, the diameters of most of the oocytes were 140–160 μm in September. The number of adults drastically decreased from September to October, suggesting that most of the adults died after spawning in the major reproductive period, although a small fraction of adults seemed to reproduce earlier or later. A cohort of recruits with an approximately 1.2 mm WS was identified in March and April. Most of the recruits grew to adult size in July–September, when many ovigerous females occurred. These results indicated that this species is fundamentally semelparous, with longevity around 1 year, and they mostly reproduce in September, with a few adults reproducing almost throughout the year.