Codium spongiosum is a species of seaweed in the Codiaceae family. The light green thallus has an applanate to pulvinate habit and is usually around 10 to 25 centimetres (3.9 to 9.8 in) across and 2 centimetres (0.8 in) thick. It has irregular lobes and moderately firm while alive becoming more spongy once dead. The form in large clusters and are 2 to 6 millimetres (0.079 to 0.236 in) long and coated in many hairs. The species is similar to Codium lucasii which adheres more tightly to rock surfaces.