The under-sea quake hit at 08:46 am (2046 GMT Saturday), 139 kilometres (86 miles) from the town of Neiafu, on the south coast of the island Vava'u, at a depth of 205 kilometres, the US Geological Survey (USGS) said.
The Pacific Tsunami Warning Centre said based on the depth of the quake, no tsunami was expected.
The epicentre of the tremor was 353 kilometres north of the capital Nuku'alofa.
Tonga, almost 2,000 kilometres northeast of New Zealand, lies on the so-called Pacific "Ring of Fire", where continental plates collide causing frequent seismic activity.