Named "Pelagornis sandersi," the bird lived 25 to 28 million years ago after the dinosaurs were wiped out but long before the first humans arrived.
The creature was an extremely efficient glider, with long slender wings that helped it stay aloft despite its enormous size.
"The specimen was so big they had to dig it out with a backhoe. The upper wing bone alone was longer than my arm," said author Dan Ksepka of the National Evolutionary Synthesis Center in Durham, North Carolina.
The new fossil was first unearthed in 1983 near Charleston, South Carolina, when construction workers began excavations for a new terminal at the Charleston International Airport.
With an estimated 20-24-foot wingspan, the creature surpassed size estimates based on wing bones from the previous record holder - a long-extinct bird named "Argentavis magnificens".
It was twice as big as the Royal Albatross - the largest flying bird today. P sandersi was probably too big to take off simply by flapping its wings and launching itself into the air from a standstill.
It may have gotten off the ground by running downhill into a headwind or taking advantage of air gusts to get aloft, much like a hang glider.
Once it was airborne, the bird's long, slender wings made it an incredibly efficient glider.
"By riding on air currents that rise up from the ocean's surface, P sandersi was able to soar for miles over the open ocean without flapping its wings, occasionally swooping down to the water to feed on soft-bodied prey like squid and eels," researchers noted.
The findings appeared in the journal Proceedings of the National Academy of Sciences.