Polysporangiophytes, also called polysporangiates or formally Polysporangiophyta, are plants in which the spore-bearing generation (sporophyte) has branching stems (axes) that bear sporangia. The name literally means 'many sporangia plant'. The clade includes all land plants (embryophytes) except for the bryophytes (liverworts, mosses and hornworts) whose sporophytes are normally unbranched, even if a few exceptional cases occur. While the definition is independent of the presence of vascular tissue, all living polysporangiophytes also have vascular tissue, i.e., are vascular plants or tracheophytes. Extinct polysporangiophytes are known that have no vascular tissue and so are not tracheophytes.

Paleobotanists distinguish between micro- and megafossils. Microfossils are primarily spores, either single or in groups. Megafossils are preserved parts of plants large enough to show structure, such as stem cross-sections or branching patterns.

Dawson, a Canadian geologist and paleobotanist, was the first to discover and describe a megafossil of a polysporangiophyte. In 1859 he published a reconstruction of a Devonian plant, collected as a fossil from the Gaspé region of Canada, which he named Psilophyton princeps. The reconstruction shows horizontal and upright stem-like structures; no leaves or roots are present. The upright stems or axes branch dichotomously and have pairs of spore-forming organs (sporangia) attached to them. Cross-sections of the upright axes showed that vascular tissue was present. He later described other specimens. Dawson's discoveries initially had little scientific impact; Taylor et al. speculate that this was because his reconstruction looked very unusual and the fossil was older than was expected.

From 1917 onwards, Robert Kidston and William H. Lang published a series of papers describing fossil plants from the Rhynie chert – a fine-grained sedimentary rock found near the village of Rhynie, Aberdeenshire, now dated to the Pragian of the Lower Devonian (around 413 to 411 million years ago). The fossils were better-preserved than Dawson's, and showed clearly that these early land plants did indeed consist of generally naked vertical stems arising from similar horizontal structures. The vertical stems were dichotomously branched with some branches ending in sporangia.

Since these discoveries, similar megafossils have been discovered in rocks of Silurian to mid-Devonian age throughout the world, including Arctic Canada, the eastern US, Wales, the Rhineland of Germany, Kazakhstan, Xinjiang and Yunnan in China, and Australia.

As of 2019^[update], Eohostimella, dated to the Llandovery epoch (443 to 433 million years ago), is one of the earliest fossils that has been identified as a polysporangiophyte. Fossils assigned to the genus Cooksonia, which is more certainly a polysporangiophyte, have been dated to the succeeding Wenlock epoch (433 to 427 million years ago).

The concept of the polysporangiophytes, more formally called Polysporangiophyta, was first published in 1997 by Kenrick and Crane. (The taxobox at the right represents their view of the classification of the polysporangiophytes.) The defining feature of the clade is that the sporophyte branches and bears multiple sporangia. This distinguishes polysporangiophytes from liverworts, mosses and hornworts, which have unbranched sporophytes each with a single sporangium. Polysporangiophytes may or may not have vascular tissue – those that do are vascular plants or tracheophytes.^{[citation needed]}

Prior to that, most of the early polysporangiophytes had been placed in a single order, Psilophytales, in the class Psilophyta, established in 1917 by Kidston and Lang. The living Psilotaceae, the whisk-ferns, were sometimes added to the class, which was then usually called Psilopsida.