Rupa bentuk rumpai laut sedikit merupai non-arboreal terrestrial plants.

• thallus: badan algal
  • lamina: suatu struktur leper seperti daun
    • sorus: spore cluster
    • pada Fucus, air bladders: float-assist organ (on blade)
    • pada kelp, floats: float-assist organ (between lamina and stipe)
  • stipe: a stem-like structure, may be absent
  • holdfast: specialized basal structure providing attachment to a surface, often a rock or another alga.

The stipe and blade are collectively known as fronds.