The United States Geological Survey said the quake struck on Tuesday at a depth of 237km.
Earthquakes at shallower depths are felt more strongly at the surface.
The jolt was centred at sea, 153km west of Neiafu, the second-largest town in the island nation.
There were no immediate reports of damage.
Tonga's National Disaster Risk Management Office warned all in the low-lying island nation to move immediately to higher ground or inland.
People should avoid beaches, shorelines, and low-lying coastal areas until an all-clear was given, said a post on the office's Facebook page.
The USGS did not issue a tsunami warning for the region, and the Pacific Tsunami Warning Center in Hawaii said in a bulletin that there was no tsunami threat because the quake was "located too deep inside the earth".
The Joint Australian Tsunami Warning Centre also said the quake on Tuesday posed no tsunami threat, as did disaster management officials in New Zealand.
Tonga is an archipelago in Polynesia made up of 171 islands with just more than 100,000 people, most of whom live on the main island of Tongatapu.
Tuesday's quake was centred nearer to the Vava'u island group.
A person who answered the phone at the Tanoa International Dateline Hotel on the beachfront at Nuku'alofa, which is on Tongatapu, said she was not aware of any damage.
"The whole building shaked. No further damage. Everything was OK," she said.
Tonga is on the Pacific Ring of Fire, the arc of seismic faults where much of the world's earthquake and volcanic activity occurs.
A tsunami set off by a volcanic eruption in 2022 killed three people.
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