Their population had dwindled to less than 1,500 before hunting of humpbacks was banned worldwide in 1966.
"It's not a complete success, but it's definitely very encouraging in terms of the recovery of the species," said Jeff Walters, co-manager of the Hawaiian Islands Humpback Whale National Marine Sanctuary.
The study, sponsored by the National Oceanic and Atmospheric Administration, is the most comprehensive analysis ever of any large whale population, said David Mattila, science coordinator for the sanctuary.
At least half of the humpback whales migrate between Alaska and Hawaii, and that population is the healthiest, Mattila said.
But isolated populations that migrate from Japan and the Philippines to Russia are taking a longer to recover after whaling operations ceased, he said.
"Whales are long-lived and give birth one at a time .... so if the population gets pushed too low, it may take quite awhile to come back. Maybe that's what's happening in the west," Mattila said.
The whales are protected under federal laws that include the Marine Mammal Protection Act and the Endangered Species Act.
Their resurgence could spark a debate over whether they should still be considered endangered, said Naomi McIntosh, superintendent for the humpback sanctuary.
"Those discussions are bound to happen, and we knew that going into the study, we anticipated it," she said. "I think it's too early to make that call."
The number of collisions between whales and boats has been increasing, probably because the population is larger, Walters said. Whale entanglements in marine debris, fishing gear and aquaculture structures also are a growing concern.
The whale count was made based on data collected from Hawaii, Mexico, Asia, Central America, Russia, the Aleutians, Canada and the United States' northwest coast.
The study used a system of photographing whale flukes -- the lobes of a whale's tail -- in six different feeding and breeding areas around the world, and then matching the pictures with whale flukes photographed in wintering areas.