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".

The bird was spotted trying to take off by running along the northbound A1(M) in Hertfordshire.
As vehicles swerved to avoid the determined swan, officers were called to rescue the bird, which was eventually persuaded to move to a petrol station forecourt.
Police drove it to a nearby lake.
More on this and other stories from Hertfordshire
The intrepid bird chose junction 10 of the A1 near Baldock "adjacent to the BP petrol station", to attempt to launch itself into the air last week, a police spokeswoman said.
"PC Paul Warner caught the swan, with assistance from PC Michael Macleod and PC Philip Watts.
"It was put into the back of the police car and driven (apprehensively) by PC Justin Smyth to a lake at Astwick Bury Farm [near Hitchin] where it was released."
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.
[NEXT_CONCEPT]
A jay-walking swan was taken into custody by four police officers after trying to use a busy motorway as a runway.