A protonema (plural: protonemata) is a thread-like chain of cells that forms the earliest stage (the haploid phase) of a bryophyte life cycle. When a moss or liverwort first grows from the spore, it grows as a protonema which develops into a leafy gametophore. Moss spores germinate to form an alga-like structure called the protonema. This structure is the precursor of the leafy gametophyte generation portion of the life cycle. Protonema are characteristic of all mosses and some liverworts but are absent from hornworts.

The protonema is also the photosynthetic part of a germinating fern spore.