Brian Lima and George Harder scored two tries each for the Samoans, who used strong midfield tackling and opportunistic runs to frustrate the Italians in the rain at the Basil Kenyon stadium.
Sham Tatupu and Darren Kellett scored the balance of the tries, with Kellett adding three conversions and two penalties, but their most devastating play came from their full-back, Mike Umaga. He set up Lima's first try in the 12th minute with a rapid dash along the right wing, executing the pass the instant that the Italian defence was about to converge on him.
The victory leaves them well placed to qualify from Pool B and with a renewed sense of purpose going into their next match, against Argentina on Wednesday. "We wanted to make the quarter-final, and the guys showed today we are a force to be reckoned with," the Samoan captain, Peter Fatialofa, said.
The Italians, whose performance must have come as an enormous disappointment in the wake of their recent victory over Ireland, scored two tries - through wing Marcello Cuttitta and full-back Paulo Vaccari.
The remainder of the Italian points, in the form of a drop goal, penalty and conversion, came from the scrum-half, Diego Dominguez.
Now Italy must steel themselves to meet the other team in the group, England, in Durban, later on Wednesday.
Western Samoa: M Umaga (Wellington); B Lima (Marist), T Vaega (Moataa), T Fa'amasino, G Harder (Marist); D Kellett (Counties), T Nu'uali'itia (Auckland); M Mika (Otago), T Leiasamaivao (Moataa), P Fatialofa (Manurewa, capt), L Falaniko (Marist), D Williams (Colomiers), S Vaifale (Marist), S Tatupu (Auckland), J Paramore (Manurewa).
Italy: P Vaccari (Milan); M Ravazzolo (Calvasino), I Francescato (Treviso), M Bonomi, Marcello Cuttitta (both Milan); D Dominguez, A Troncon (both Milan); Massimo Cuttitta (Milan, capt), C Orlandi (Piacenza), F Properzi Curti, P Pedroni (both Milan), R Favaro (Treviso), C Checchinato (Rovigo), J Gardner (Roma) O Arancio (Catania).
Referee: J Dume (France).Reuse content