Since the Late Paleozoic, forests have become distributed worldwide and significantly changed the Earth's climate and landscapes, but the record of forests is rare in the Devonian (419-359 Ma in age) when they first appeared. From the Upper Devonian (Famennian with the age of 372-359 Ma) of Xinhang, Anhui, China, we report a very large in situ forest, which includes locally dense stands of lycopsid plants. The Xinhang forest is monospecific with a small tree lycopsid Guangdedendron gen. nov., probably dioecious with monocarpic reproduction. The plant shows the earliest stigmarian rooting system typical of giant tree lycopsids dominating Carboniferous forests. It colonizes coastal clastic wetlands that were influenced by floods. This significantly increases the paleogeographical coverage of in situ Devonian forests, and contributes to our understanding of atmospheric CO2 decline and coastal consolidation.