The tremor struck at 1602 GMT around 100 kilometers (60 miles) off Tonga's far off volcanic island of Niuatoputapu at a profundity of 210 kilometers, the US Topographical Study said.
It stated that there was no tsunami risk.
Gary Vite, chief meteorologist at Tonga Meteorological Services near the capital Nuku'alofa, stated, "We had calls early in the morning from people who felt it, but there have been no reports of damage."
Around 360 kilometers away from the epicenter, in Apia, the capital of Samoa, the quake was felt as well.
The Samoa Meteorological Service said that the office near Apia was shaken by "strong vibrations," but there were no reports of damage.
Tonga and Samoa are on the Pacific "Ring of Fire," where tectonic plates frequently shift and earthquakes frequently occur.