Researchers studied more than 1,000 fossils of the Montsechia Vidalii species as part of the study.
The plant resembled a pond-weed but bore fruit containing a single seed - the defining characteristic of a flowering plant.
The scientists say it grew in Spanish lakes more than 125 million years ago.
"A 'first flower' is technically a myth, like the 'first human'," botanist David Dilcher says in the Proceedings of the National Academy of Sciences.
"But based on this new analysis, we know now that Montsechia is contemporaneous, if not more ancient, than Archaefructus," a similar aquatic plant found in China.
Scientists have known about the Montsechia Vidalii species for a long time.
Its fossils were first discovered more than a century ago in the limestone deposits of the Iberian Range in central Spain and in the Montsec Range of the Pyrenees.
But Mr Dilcher said many of the fossils were misinterpreted, because Montsechia "possesses no obvious 'flower parts' such as petals or nectar-producing structures for attracting insects".
Botanists in the US say an ancient plant that grew underwater in what is modern day Europe may have been the world's first known flowering plant.