Modiolus barbatus can grow up to 6 cm long, although individuals are usually smaller. The shell is an elongate oval shape with the hinge line at a steep angle to the prominent, rounded umbones. The beaks are not quite at the anterior end of the shell. The outer surface of the shell is yellowish-white, light yellow or reddish-brown in colour. Over the posterior half of the shell the periostracum bears long, flat bristles, each with a distinctly serrated edge. The inner surface of the shell is shiny and pale blue in colour.