The exquisitely preserved remains were unearthed in a US coalmine in Illinois, and date back to 300 million years ago.
Writing in the journal Geology, a UK-US team said a diverse array of now extinct fossilised flora could be seen.
Covering a 1,000-hectare (10 sq km) site, it represents the largest fossil rainforest yet discovered.
"This is a significant find," said Professor Andrew Scott, a palaeobotanist from Royal Holloway University, London.
"These peat-forming systems are actually quite diverse and dynamic.
"What they are showing is that this ecological diversity is also seen spatially."
Ancient insights
The Pennsylvanian Epoch, which dates to 299-318 million years ago, is considered the "zenith of peat formation".
Geologists are keen to understand more about the period to gain insights into the formation of coal.
"We were able to make a map of the forest by the light of our miner's lamps"
The deposit in Illinois is thought to have been created when a major earthquake shook the forest, causing a vast swathe to drop below sea level and become preserved as peat and then coal.
Dr Howard Falcon-Lang, an earth scientist from the University of Bristol, was one of the scientists to discover the site. He said: "It was an amazing experience.
"We drove down the mine in an armoured vehicle, until we were a hundred metres below the surface.
"The fossil forest was rooted on top of the coal seam, so where the coal had been mined away the fossilized forest was visible in the ceiling of the mine.
"We walked for miles and miles along pitch-black passages with the fossil forest just above our heads. We were able to make a map of the forest by the light of our miners' lamps."
Species diversity
The scientists found a rich mixture of ecology. An abundance of 4m-high (12ft) tree ferns formed a sub-canopy over the forest, but they were dwarfed by club mosses, some more than 40m tall.
Dr Falcon-Lang said the discovery allowed researchers to understand how the make-up of the forest changed across the ancient landscape.
"As there is nothing like it around today, before our work we knew very little about the ecological preferences and community structure of these ancient plants," he said.
Other work is also starting to shed light on these systems.
A team from the Czech Republic recently discovered and mapped an ancient forest, from the same time period, buried under volcanic ash.
"Taken together, these two pieces of work are illuminating our understanding of how these vegetation systems work," said Professor Scott.
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