Waves up to 5m high swept several kilometres inland, entire villages and towns were wiped out, and power was knocked out in many areas, according to witnesses.
Jason Smith, the head of communications for the International Federation of the Red Cross, said: "Across Samoa, 60 villages, nearly 15,000 people have directly impacted.
"The Samoa Red Cross is working hard through five evacuation centres to provide people with safe places to stay and access to clean water," he told Al Jazeera.
The toll is expected to rise as rescue teams work their way through the worst hit areas and assess the full extent of the damage.
"It's believed as of now, there could be a number close to 100 deaths," Ausegalia Mulipola, the assistant chief executive of Samoa's disaster management office, said.
"They are still continuing the searches for any missing bodies in the area," Mulipola said, adding the southern side of the country's main island Upolu was the worst hit.
"Some areas have been flattened and the tsunami had brought a lot of sand onshore, so there have been reports the sand has covered some of the bodies. So we need specialised machines to search for bodies that are buried under the sand."
Samoa's deputy prime minister said a resort area popular with foreign tourists had been "devastated".
Misa Telefoni added that residents and visitors had little time to prepare for the onslaught.
"People were saying that there was the shake and the ocean went out within five minutes, so that's pretty fast and that makes it extremely difficult," he said.
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.
"We've heard that most of the resorts are totally devastated on that side of the island," he said. "We've had a pretty grim picture painted of all that coast."
Tuilaepa Sailele Malielegaoi, Samoa's prime minister, said he was "shocked beyond belief".
"So much has gone. So many people are gone," he told the Australian news agency AAP. "I'm so shocked, so saddened by all the loss."
But fears of a devastating ocean-wide tsunami were calmed after the Pacific Tsunami Warning Centre cancelled its warning for the region three and a half hours after issuing it.
Guy Urban, a geophysicist with the West Coast and Alaska Tsunami Warning Centre, told Al Jazeera that strong currents were still expected as far away as California in the US western coast, as well as Australia and New Zealand.
Residents of Auckland in New Zealand moved to higher ground but the authorities later cancelled the alert as 40cm waves arrived, causing little damage.
Japan's meteorological agency issued its own tsunami warning along its entire eastern coast, warning that waves could reach 50cm high.
Jonah Tuiletufuga from Radio Polynesia told Al Jazeera that "there was a great response from the public".
"For the last three years there has been a drill implemented so there are designated areas where you have to run or walk to in the event of a tsunami."
Barack Obama, the US president, declared "a major disaster" in American Samoa and ordered federal aid to supplement local recovery efforts, the White House said.
The means immediate resources for emergency response measures would be made available as well as federal funding for the territory.
The US Federal Emergency Management Agency said it was dispatching two emergency teams to the region, though it was unclear how quickly its personnel would arrive.