The All Black Sevens have had a disappointing Paris tournament after they were beaten in the semi-finals of the Plate competition.
The New Zealanders were beaten 19-17 by Argentina in the quarter-finals, before going down to South Africa 29-19 in the Plate semi-final.
Samoa came back from 19 points down to beat Fiji 29-26 in the final. Despite the loss Fiji extend their lead in the World Series standings.
Fiji has 166 points, South Africa 152 and New Zealand 145.
The final round is in London this weekend.
It is Samoa's first World Series tournament win since December 2012 in Dubai and gives them confidence and momentum heading into the final round in London, and the Olympic repechage qualifier in Monaco on June 18-19.
Fiji dominated the opening 10 minutes of the final and led 26-7 at the break through tries by Isake Katonibau, Leone Nakarawa and Jerry Tuwai (2).
But Belgium Tuatagaloa clawed one back and after the break Tila Mealoi (2) and Siaosi Asofolau brought the Samoans roaring back before captain Faalemiga Selesele's try capped a remarkable comeback.
"It's a huge surprise, especially after Fiji were so strong against us yesterday (winning 42-5). We wanted to get some pride back and we did that," said Samoa coach Damian McGrath. "At half time I said if we get the first score we can get back into this. Self belief is a huge thing, and they believed in themselves."
Despite the loss, Fiji have taken a giant step towards securing the overall 2015-16 series title.
They now lead the standings by 14 points ahead of South Africa with just one round remaining, with New Zealand third.
The pools for this weekend's final leg in London have been made with Cup winners Samoa heading pool A and facing matches against South Africa, USA and Canada.
Paris runners-up Fiji face Australia, England and Wales in pool B, France start against Kenya, Scotland and Portugal, while Argentina head pool D with matches against New Zealand, Russia and Olympic hosts Brazil.