An impression of what the flower may have looked like
It was discovered in a slab of stone in north-east China and the plant is thought to have lived at least 125 million years ago.
They say it probably grew in shallow lakes shared by dinosaurs and other prehistoric creatures.
The plant, called Archaefructus sinensis or "ancient fruit from China", is of a species never before seen, says David Dilcher, of the Florida Museum of Natural History and the University of Florida.
"It is like the mother of all flowering plants," he adds.
Modern relative
Professor Dilcher is the co-author of a study published on Friday in the journal Science. He worked with Ge Sun, a geologist at Jilin University in Changchun, China, and other researchers.
Botanists had long considered a woody plant from New Caledonia as the most ancient of flowering plants. But Professor Dilcher said the new discovery was even older.
"It changes our whole impression of what is the oldest of all flowering plants," he said.
The flower's closest "modern relative" was probably the water lily, said Professor Dilcher, because it apparently lived in clear, shallow waters, with its flowers and seeds extending above the surface.
Some scepticism
The discovery suggested that flowering plants started out as herbs that were able to reproduce quickly, he said.
It "was not a flashy flower," he said. The plant's flowering part had no real petals, but acted only as a reproductive unit - essential for its survival.
"The reason we can say it is a flowering plant is that the seed is enclosed inside of carpels [female part] of the fruit," said Professor Dilcher.
Other experts in Science said more research was needed before the new flower was generally accepted as the most ancient of flowering plants.
But Peter Raven, of the Missouri Botanical Garden in St Louis, said it "may be the most significant flowering plant ever found".