Buildings were toppled and thousands of people fled to higher ground after the offshore quake struck in the early morning, followed by giant waves which swept cars out to sea.
At least 22 were confirmed dead in American Samoa, while the Red Cross said another 14 were dead in Samoa and warned that the death toll was expected to rise amid unconfirmed reports of scores missing.
Eyewitnesses reported walls of water of between three and nine metres (yards) pounding the shore, wiping out villages and shattering holiday resorts.
Samoa's deputy prime minister Misa Telefoni said a resort area popular with foreigners was "devastated" by the tsunami that followed the monster quake and that residents and holidaymakers had little time to flee.
"We've heard that most of the resorts are totally devastated on that side of the island. We've had a pretty grim picture painted of all that coast," he said.
Two of the country's most popular resorts, Sinalei Reef Resort and Coconuts Beach Resort, off the west coast of the main island of Upolu, had been hit hard, he told the Australian Associated Press.
There was widespread destruction in Samoa with possibly thousands of people left homeless on the island, local journalist Jona Tuiletufuga told AFP.
"We are getting reports of missing people in areas where damage is extensive on the south and southeast coasts," he told AFP. "Entire villages have been wiped out."
Tuiletufuga said there were up to 70 villages in the worst-hit area and each housed from 300-800 people.
Apia, capital of the independent state of Samoa and nearly 3,000 kilometres (1864 miles) from Auckland in New Zealand, was evacuated as officials scrambled to get thousands of residents to higher ground.
Officials in American Samoa, about 100 kilometres (60 miles) from Samoa, said at least 22 people were dead and that the toll was expected to climb.
"It could take a week or so before we know the full extent," Michael Sala, Homeland Security director in American Samoa, told AFP.
Waves measuring around 25-feet (7.5-metres) high did most of the damage as they swept ashore about 20 minutes after the earthquake, demolishing buildings in coastal areas, he said.
Witnesses said cars were swept out to sea in American Samoa, where buildings were destroyed in what the US territory's Congress delegate said was a scene of "devastation."
The eastern part of the island was without power and water supplies after the devastating earthquake, which struck at 6:48 am (1748 GMT) at a depth of 18 kilometres (11 miles), 195 kilometres south of Apia.
The Pacific Tsunami Warning Centre said waves of up to 1.57 metres (over five feet) had smashed into American Samoa. It issued a tsunami alert over a vast swathe of the Pacific, as far as Hawaii, which was later cancelled.
Australia said at least one of its citizens was feared dead and six others missing, while a New Zealander was also feared dead.
Australia's foreign ministry also said it had reports of death and injuries in the Tonga and the Niua islands after the tsunami hit, while two South Koreans were among the dead, Seoul officials said.
Australia and New Zealand both made preparations to send emergency help to the ravaged region that is home to more than 241,000 people.
Mike Reynolds, superintendent of the National Park of American Samoa which is headquartered in the territory's capital Pago Pago, told colleagues in California there had been widescale destruction.
National Park Service spokeswoman Holly Bundock said she had spoken to Reynolds, who was sheltering under a coconut tree.
"They said five tsunami waves have hit the park visitor centre in Pago Pago. It would appear park offices and the visitor centre there have been destroyed. "One of the waves was about 30 feet high," she said