Lofipo was named for her maiden Jillaroos squad on Monday, as part of an international tug-of-war for her services this season.
But the Gold Coast star told Jillaroos officials she instead wanted to remain loyal to Samoa, after debuting for the country under Jamie Soward in 2023.
"I'm incredibly honoured and grateful to be selected for the Jillaroos but after thinking long and hard and talking with my family, I want to represent Samoa again this year," Lofipo said in a statement.
"It was an extremely tough decision, but I want to uphold the commitment I made to Samoa after qualifying last year for our first Women's Rugby League World Cup in 2026.
"I want to thank Jillaroos coach Jess Skinner for considering me and I wish her and the squad well for the Pacific Championships."
Lofipo's decision comes as the Gold Coast backrower headlines Samoa's rise from as low as No.26 in the world a few years ago.
They now loom as arguably the third best team in the international women's space, after qualifying for next year's World Cup.
Soward's side will face a serious litmus test over the next month, as part of the top tier of the Pacific Championships against Australia and New Zealand.
But Lofipo's call to prioritise the Samoan jersey represents a landmark moment for the group, after her Queensland Origin debut last year and Jillaroos call up.
"I picked her years ago, she is a huge part of our younger group," Soward told AAP.
"With how well she played in Origin she is a core piece of what we're building in our ranks.
"I was super excited yesterday when she called me yesterday and said she was playing for Samoa."
Samoa also have fellow Queensland State of Origin players Destiny Brill and Tarvarna Papalii in their squad, while Taliah Fuimaono has also represented NSW.
The team has made significant ground in the past year with New Zealand sevens legend Niall Williams-Guthrie also back after featuring in last year's Pacific Bowl success.
Brisbane's Dally M Rookie of the Year Shalom Sauaso headlines the new faces in a squad captained by Annetta-Claudia Nu'uausala.
New Zealander Amber Hall was also set to make a switch to Samoa before being ruled out through injury.
"We've seen it on the boys' side how quickly it's happened," Soward said.
"For the girls it's about believing that they can come and represent Samoa and we've got a qualify program."
SAMOA SQUAD FOR PACIFIC CHAMPIONSHIPS:
Annetta-Claudia Nu'uausala, Belinda-Jane Likah Clarke, Destiny Brill, Destiny Mino-Sinapati, Eliza Lopamua, Ella-Jaye Harrison-Leaunoa, Estanoa Faitala-Mariner, Jessica Patea, Jetaya Faifua, Lindsay Tui, Mercedez Tauleitei-Siala, Niall Williams-Guthrie, Pauline Pilaie-Rasable, Pihuka Berryman-Duff Ryvrr-Lee Alo, Sarina Masaga, Shalom Sauaso, Sienna Lofipo, Simone Karpani, Taliah Fuimaono, Tarvarna Papalii.
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