The Archaeplastida are a major line of eukaryotes, comprising the land plants, green and red algae, and a small group called the glaucophytes. All of these organisms have plastids surrounded by two membranes, suggesting they developed directly from an endosymbiotic cyanobacteria. In all other groups, plastids are surrounded by three or four membranes, and were acquired secondarily from green or red algae.

The cells typically lack centrioles and have mitochondria with flat cristae. There is usually a cell wall including cellulose, and food is stored in the form of starch. However, these characters are also shared with other eukaryotes. The main evidence the Archaeplastida form a monophyletic group come from genetic studies, which indicate that plastids probably had a single origin.

The archaeplastids fall in two main evolutionary lines. The red algae are pigmented with chlorophyll a and phycobilins, like most cyanobacteria. The green algae and land plants (Viridiplantae) are pigmented with chlorophylls a and b but lack phycobilins. The positions of the glaucophytes are uncertain; they have the typical cyanobacterial pigments, and are unusual in retaining a cell wall within the plastids (called cyanelles).

Cavalier-Smith (1981) suggested that the kingdom Plantae should refer to this group, and accordingly it may be called the Plantae sensu lato, but this has not been generally adopted, partly because of questions over its monophyly. The more precise name Archaeplastida was introduced by Adl et al. (2005).

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