Replacement hooker Seilala Lam scored two tries following his introduction at the interval as Samoa overturned a 17-3 deficit to notch up a third successive victory.
"It took an 80 minute effort to get there, we know the Fijians are world class and they showed that in the first half," said Samoa captain Michael Ala'alatoa.
"I'm so proud of our boys to hang deep. In the past we would've lost those games but we're starting to build a winning culture."
Needing to win to claim the title, Samoa looked like they were going to fall short as Fiji took control following a drive over the line from Albert Tuisue and an individual effort by Frank Lomani that left three defenders in his wake.
However, Lam rolled off the back of a maul to reduce the deficit two minutes into the second half and a pair of penalties from Rodney Iona moved Samoa to within four of the Fijians.
Lam struck once more 10 minutes from time when he again capitalised on the strength of the pack to touch down and Iona's conversion gave Samoa a match-winning cushion.
Earlier on Saturday, two tries from Filipo Daugunu helped Australia A to a 39-22 victory over Tonga.
Samiuela Moli put Tonga, who will face Hong Kong in a crucial World Cup qualifier in Australia next weekend, in front in the second minute when he touched down at the back of a driving maul.
But, after Ryan Lonergan's penalty, Daugunu dived to score in the corner in the 13th minute to give his side the lead.
Lonergan and William Havili traded penalties before the Australians extended their lead through Langi Gleeson.
Hamish Stewart added a third under the posts five minutes after the restart before Moli reduced the deficit for Tonga with a carbon copy of his opener.
Daugunu capitalised on Gleeson's bulldozing run on the hour mark to score in the corner while James Tuttle cancelled out Jay Fonokalafi's try to ensure a comfortable win.
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