The X-43A aircraft on Tuesday flew at a speed of about Mach 9.6 - nearly 10 times the speed of sound - after a booster rocket took it to around 110,000 feet and then separated.
A modified B-52 airplane had carried the experimental plane and booster aloft.
It was the last of three test launches for the X-43A series and its supersonic-combustion ramjet or "scramjet" engine. The scramjet scoops up oxygen from the air rather than carrying liquid oxygen in a tank like an ordinary rocket.
Scramjet technology, Nasa has said, could open the way to cheaper, safer and faster flights into the upper atmosphere, with smaller and lighter craft.
"I think it's easier than people think it is. We can really do this stuff. I don't mean to make it sound too easy, but it's definitely doable," said Randy Voland, a senior research engineer on the project, at a news conference after the test.
The eight-year, $230 million programme got off to a rough start in June 2001 when the first X-43A and its booster rocket had to be destroyed in mid-air. The second attempt, in March of this year, successfully reached a speed of Mach 7.
How it went
That Mach 7 flight set the previous world record for a jet-powered vehicle, Nasa said.
The silvery-black scramjet, just 12 feet long by five feet wide, took off from Edwards Air Force Base in the desert north of Los Angeles perched below Nasa's B-52 in the early afternoon on Tuesday.
Engineer Randy Voland says it is
easier than people think
After reaching launch altitude over the Pacific, the modified bomber dropped the scramjet and its booster rocket for a run at the speed record.
Nasa video images showed the scramjet rising sharply, powered by the booster rocket, before the booster separated at about 110,000 feet and the scramjet kicked in.
After a few seconds, the X-43A entered a glide, quickly losing speed before a crash-landing into the ocean after a total journey of about 800 miles.
Along the way, the scramjet was expected to encounter temperatures of about 3000 degrees Fahrenheit, roughly one-third hotter than the test in March that reached Mach 7.
Engineers said the scramjet cruised after the separation, neither gaining nor losing speed during its operation. The 20 seconds of operation, they said, gave them far more research than they have had before on jet functions at those speeds.
"We have quite a lot to look at for quite a long time to come," said Laurie Marshall, chief engineer on the flight.
Nasa said it had no plans to recover the craft, which has been standard procedure with the scramjet tests.
The flight on Tuesday had been delayed from the previous day owing to electronic problems.