Samoa's 38-16 defeat to Fiji at the Pacific Nations Cup, which featured a hat-trick from Henry Seniloli, ensured they can no longer catch Tonga in the standings.
That leaves Tonga as the second Oceania qualifier and they will join England, France, Argentina and the United States in Pool C in the tournament in Japan in two years' time.
Tonga beat both Italy and the United States in last year's Autumn Internationals.
For the 2015 Rugby World Cup, Tonga were drawn in New Zealand's pool, finishing 4th out of the five teams. They lost 47-9 to the eventual champions.