The monster quake hit 157 miles (256km) southeast of Chiniak, Alaska, at a depth of 10 km at 9.31am GMT, the US Geological Survey (USGS) said.
The earthquake in the Gulf of Alaska was about 400 miles (578km) south of Anchorage, which is Alaska’s most populous city with a population of nearly 300,000.
Anchorage residents have been urged to move away from the coast amid warnings of an “extraordinary threat to life or property”.
A tsunami watch is in effect for Washington State, Oregon, California, Hawaii and parts of Mexico, according to the Tsunami Warning Centre.
In California, tsumani warnings were put in place from the coast from the Californian/Mexico border to the Oregon/Californian border including San Francisco Bay.
WPTV Network issued the warning and said: “The Associated Press reports an 8.2 magnitude earthquake hitting off the Gulf of Alaska. A ‘watch’ warning has been issued for California, Washington and Oregon.
“And the big concern at this point is the tsunami warning that’s been put in effect for much of California and that includes the San Francisco Bay area.
“There’s no information at this point as far as any damage that this earthquake may have caused.
“But it’s something we are going to be keeping an eye on as this morning continues on.”
San Francisco officials tweeted an urgent message: “If you are w/in SF & 3 blocks of the Pacific Coast or w/in 5 blocks of SF Bay, PREPARE TO EVACUATE SO YOU ARE READY IF EVACUATION IS NEEDED. Check on neighbors who may need help.”
Alaska is situated on the Ring of Fire, a huge area in the basin of the Pacific Ocean where earthquakes and volcanic eruptions frequently occur.
The quake comes just hours after a 6.1-magnitude earthquake struck off the coast of Java, Indonesia, sending people in the capital running into the streets.
Residents living in California can use their addresses to check whether they are living in a tsunami danger zone following the Alaska earthquake here.