Plant body of algae, fungi (see fungus), and similar simple, plantlike organisms. Composed of filaments or plates of cells, a thallus ranges in size from a single-celled structure to a complex treelike form. The photosynthetic and supportive cells tend to be organized linearly, but a thallus lacks such differentiated specialized structures as stem, leaves, and conducting tissue. Most thallus plants are currently classified as complex protists.

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For the chronologer/historian occasionally mentioned in the works of early Christian writers, see Thallus (historian).

Thallus, from Latinized Greek θαλλος (thallos), meaning a green shoot or twig, is an undifferentiated vegetative tissue of some non-mobile organisms, which were previously known as the thallophytes.

An organism or structure resembling a thallus is called thalloid, thallodal, thalliform, thalline, or thallose.

Even though thalli do not have organized and distinct parts (leaves, roots, and stems) as do the vascular plants, they may have analogous structures that resemble their vascular "equivalents". The analogous structures have similar function or macroscopic structure, but different microscopic structure; for example, no thallus has vascular tissue.

Although a thallus is largely undifferentiated in terms of its anatomy, there can be visible differences and functional differences. A kelp, for example, may have its thallus divided into three regions. The parts of a kelp thallus include the holdfast (anchor), stipe (supports the blades) and the blades (for photosynthesis).

The thallus of a fungus is usually called a mycelium. In seaweed, thallus is sometimes also called frond.

The gametophyte of some non-thallophyte plants -- clubmosses, horsetails, and ferns is termed "prothallus".