Parliament was to discuss Mr Sovaleni's leadership on Monday with 'Aisake Eke poised to bring on the motion on behalf of 10 MPs.
However, Mr Sovaleni surprised observers by offering his resignation in a parliamentary address, doing so with tears in his eyes, according to local outlet Matangi Tonga.
"I'll be resigning immediately according to the constitution," Mr Sovaleni said.
It is not yet clear who will replace Mr Sovaleni, who was Tonga's leader for three - sometimes difficult - years.
Last year, he survived a separate no-confidence vote, while in April, he resigned as defence minister when King Tupou VI withdrew confidence.
Mr Sovaleni told Radio NZ "it's just better for me to leave" and "this is the best way forward for Tonga".
Tonga was also the host of this year's Pacific Islands Forum (PIF), making Mr Sovaleni the chair of the pivotal regional organisation, which rotates that position among Pacific leaders.
Tonga hosted this year's Pacific Islands Forum. (Lukas Coch/AAP PHOTOS)
The Polynesian nation hosted PIF leaders meeting in August, when Australian Prime Minister Anthony Albanese and other heads of government visited Nuku'alofa.
Despite its relatively small size, and a nearby earthquake on the opening day of the summit, Tonga pulled off a well-organised event, noted by all for warm hospitality.
"The community, the church and that have all come together to provide hospitality to our visitors, our forum members and dialogue partners," Mr Sovaleni said.
"I'm very happy we've had that support welcoming the Pacific and the world here to Tonga."
Mr Sovaleni will address local media later on Monday.
31°C