The search was called off at 3:30 p.m. after a final attempt to locate Richard Abruzzo, of Albuquerque, N.M., and Carol Rymer Davis, of Denver, had failed, said coast guard spokesman Lt. Massimo Maccheroni.
Maccheroni said that a robotic vehicle scanned the seabed of the Adriatic for any remains.
"We found nothing that could be traced to the balloonists," he told The Associated Press.
The veteran pilots were flying in the 54th Gordon Bennett Gas Balloon Race when contact was lost Wednesday over the Adriatic Sea. Race organizers said the two plunged toward the water at 50 mph and likely didn't survive.
Since then, search and rescue teams with the Italian coast guard, the U.S. Navy and Croatian coastal aircraft crews have been scouring the Adriatic Sea. Over the weekend, divers joined in the search, as hope was beginning to fade.
On Monday, in a last attempt, the robotic vehicle plunged to depths of 656 feet to photograph the seabed off Vieste, in Puglia, where the balloon was believed to have crashed.
Maccheroni said the robot scanned an area where an aircraft a day earlier had spotted something. But nothing related to the balloon or the pilots was found, he said. Strong sea currents can drag both relics and bodies very far away from a presumed point of impact, Maccheroni said.
News that the search had ended quickly reached the Albuquerque International Balloon Fiesta in New Mexico.
"Everybody is getting into the fiesta now and trying to move forward. I think we have accepted the fact they're probably not coming back," said Troy Bradley, who with Abruzzo in 1992 piloted the first balloon to fly from North America to Africa.
Abruzzo, 47, and Davis, 65, won the 2004 edition of the Gordon Bennett race and the 2003 America's Challenge gas race one of Abruzzo's five victories in that race.
Abruzzo worked as part of a prominent family business in Albuquerque that is involved in real estate and operations of the Sandia Peak tramway, Sandia Ski Area and Ski Santa Fe. Richard Abruzzo's involvement focused on ski area management.
Davis was a radiologist who specialized in reading breast mammograms.
The decision to call off the search was made by Rear Adm. Salvatore Giuffre, who had been coordinating the search efforts in southern Italy.