Thallus is an undifferentiated vegetative tissue (without specialization of function) of some non-mobile organisms, which were previously known as the thallophytes.

Even though thalli do not have organized and distinct parts (leaves, roots, and stems) like 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 conducting tissue .

The thallus of fungus has been also called soma since late 20th century, but the alternative name is still uncommon. In seaweed, thallus is sometimes also called frond.

The gametophytes of some non-thallophyte plants -- clubmosses, horsetails, and ferns -- are also termed "thallus".

"Thallus" is Latinized Greek, thallos, meaning "a green shoot/twig". An organism or structure resembling a thallus is called thalloid, thallodal, thalliform, thalline, or thallose.