Portion of the of various organisms employed to adhere to surfaces.

Holdfasts are components of numerous , . In understanding holdfasts, it is important stress that they are used literally to "hold" organisms ("fast") to surfaces and thus serve to facilitate that hold rather than to either obtain nutrients, including for the rest of the organism, or instead to disrupt those surfaces any more than is required to effect holding on.

Though similar as a compoent of an algal thallus to the of plants in the sense that they are used to anchor those algae at their bottom, algae do not similarly require access through their holdfasts to water and (since algae otherwise are completely surrounded by both water and minerals). Note that attached to the algal holdfast is the algal stipe.