The US Geological Survey said the epicentre of the 8.3 magnitude temblor was 204km southwest of Apia at a depth of 85km.
A tsunami warning was issued for Niue Island, the Wallis and Futuna Islands, the Tokelau atolls, the Cook Islands, Tuvalu, Kiribati, the Kermadec Islands, the Baker and Howland Islands, Jarvis Island, French Polynesia, the Palmyra Islands as well as Samoa and American Samoa.
"An earthquake of this size has the potential to generate a destructive tsunami that can strike coastlines near the epicenter within minutes and more distant coastlines within hours," a statement from the Pacific Tsunami Warning Centre said.
"Authorities should take appropriate action in response to this possibility," the statement said.
However, there was no immediate word of any casualties or damage from the quake or the tsunami.
Keni Lesa, a resident of Apia, told the AFP news agency: "I'm taking my family to a safe place. Everyone's getting out of coastal areas."
But he said that there was no panic as "we have done a lot of training for this".