Lome Fa’atau, Alesana Tuilagi and Kane Thompson grounded before the break as the Polynesians avoided the ignominy of going through the tournament winless, following earlier defeats to South Africa, Tonga and England.
That fate could still befall the US, who fought back gamely from a 22-3 interval deficit to grab second-half tries through Takudzwa Ngwenya and Louis Stanfill with Mike Hercus kicking 11 points.
Samoa, whose other points came through full-back Loki Crichton via two penalties and two conversions, deserved their belated win, with fly-half Eliota Fuimaono-Sapolu the stellar player on show.
They were 12-0 ahead after just nine minutes. Fa’atau capitalised on turnover ball to score in the corner.
Tuilagi then scrambled over before Thompson finished off smart work by the inspired Fuimaono-Sapolu just before half time.
Reduced to 14 men when Fifita Mounga was yellow-carded, the US seemed to be spurred on. Ngwenya put on the accelerator to motor over in the right corner and a Hercus penalty in the 65th minute reduced the arrears to nine points.
Crichton steadied the Samoa ship with a three-pointer of his own with nine minutes remaining, but Hercus’ 75th-minute penalty kept it relatively close.
Samoan Silao Vaisola Sefo was sin-binned late on, and they conceded a late try to Stanfill. But by then the game was over.
Samoa scorers: Tries: Fa’atau, A. Tuilagi, Thompson. Cons: Crichton 2. Pens: Crichton 2.
Crichton, Fa’atau, Savealii, Mapusua, A. Tuilagi, Fuimaono-Sapolu, Poluleuigaga, Lealamanua, Schwalger, Johnston, Lafaiali’i, Thompson, Sititi, Purdie, Vaeluaga.
Replacements: Lemi for Fa’atau (60), Lui for Fuimaono-Sapolu (70), Sooialo for Poluleuigaga (70), Leleimalefaga for Lealamanua (70), Sefo for Schwalger (73), J. Tekori for Thompson (64), Ulia for Vaeluaga (67).
USA scorers: Tries: Ngwenya, Stanfill. Cons: Hercus. Pens: Hercus 3
Wyles, Ngwenya, Eloff, Esikia, Sika, Hercus, Erskine, MacDonald, Lentz, Osentowski, Parker, Mexted, Stanfill, Clever, Mo’unga.
Replacements: Tuipulotu for Esikia (73), Moeakiola for MacDonald (68), Burdette for Clever (45), Aylor for Mo’unga (64).
Wayne Barnes (England).