Seniloli's hat-trick and tries from centre Jale Vatubua and lock Api Ratuniyarawa ensured the Fijians went unbeaten in the Pacific Nations tournament and finished on 17 points. Tonga ended on six and Samoa on five.
Tonga will now join England, France, Argentina and the United States in Pool C.
Fiji qualified last week after they beat Tonga 14-10 in Nuku'alofa and as the top-ranked Oceania team will join Australia, Wales, Georgia and either Canada or Uruguay in Pool D in Japan.
"I want to thank all the boys for all their work over the past seven weeks," Fiji captain Akapusi Qera said.
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"It's a good feeling. It's why we play rugby. You want to win silverware and to win the PNC title is a big bonus."
Samoa, whose points came from the boot of Tusi Pisi and a try to captain David Lemi in his 50th test appearance, now face a playoff against a European side, with the loser of that game entering the repechage tournament.
"It's no excuses for us. I'd like to congratulate Fiji for their win in the PNC," Lemi said.
"We have to dig deep to make sure we qualify for the World Cup."
The Fijians also won both of their games last year, beating Tonga 23-18 and Samoa 26-16 in Suva before they qualified for Japan last week with their third win.
(Reporting by Greg Stutchbury; Editing by Nick Mulvenney)
