They will be electing 17 People's Representatives for the 26-member legislature.
The 33 noble families will elect their nine representatives from within their own ranks.
There are 72 candidates standing, including 12 women.
RNZ Pacific's correspondent in Tonga, Kalafi Moala, said a high voter turnout is expected.
"There has been leading up to the election a very strong drive to get people registered to vote. People over 21 years of age are eligible to vote here in Tonga and then of course there's been people that have returned from overseas, people that have moved from other areas.
So there has been a strong registration drive and so the expectation also is there will be a higher vote this year than the previous election," he said.
The polls have just opened and a result in expected by 10 o'clock tonight.
If this election follows the pattern of recent elections the prime minister will not be chosen until nearer Christmas.