Gelidium vagum is a red alga, often with a treelike appearance, in which one or several vertical trunk-like axes arise from a basal axis, with widely spaced short branchlet. The trunks can reach 100 mm tall, but are usually shorter. The color ranges from yellowish red through brighter red to brownish red. This seaweed is native to the Northwest Pacific, from Pacific Russia to the Yellow Sea in China. In 1984, it was collected on Denman island in British Columbia, in areas where the Pacific Oyster (Magallana gigas formally Crassostrea gigas) was cultured. In 1996, it was collected in US waters, in the San Juan Islands and Bellingham Bay. It was also found in Tomales Bay, California Bay, where it was probably transferred with Pacific Oysters from British Columbia or Washington.