Morphological studies were undertaken on Gastroclonium pacificum (E.Y. Dawson) C.F. Chang et B.M. Xia (Champiaceae, Rhodymeniales) from Japan. We describe the details of male reproductive structures, the procarp and early post-fertilization stages. This species has a solid axis, with both tetrasporangia and polysporangia, spermatangia are cut off from spermatangial parent cells, and a procarp is composed of a 4-celled carpogonial branch and two 2-celled auxiliary cell branches. The mature cystocarp lacks a conspicuous ostiole, a characteristic of the genus Gastroclonium. The most distinctive characteristic of the species is the tela arachnoidea, which is lacking in other species of Gastroclonium.