Phyllospadix iwatensis is a plant species found along the seacoasts of Japan, Korea, China (Hebei, Liaoning, Shandong), and the Russian Far East (Sakhalin, Primorye and the Kuril Islands). It was first discovered in 1929 near on the Miyako Peninsula in Iwate Prefecture in Japan, in northeastern Honshu. It occurs in the intertidal zone along the shore.