The 37-year-old led the Black Ferns to their fifth women's World Cup title last year after a pulsating final against England and had planned to hang up her boots after 52 caps in a 15-year career.
NZR, however, said in March they would offer retainer contracts worth up to NZ$50,000 (26,085 pounds) to the country's top 30 players in 15-a-side rugby for the first time, while allowing them to remain employed or in study.
Faamausaili is a police detective and Black Ferns coach Glenn Moore said the prospect of a contract had encouraged the hooker to delay her retirement.
"She is as fit as she's ever been and is super passionate about being involved," Moore said in a statement on Wednesday when announcing 28 of the 30 contracted players.
"There are a number of new faces among those named and to have Fi's skills and experience to call on will be invaluable.
"She also brings fantastic skills in her position and it's an area we need to develop with newer players. She can help us with that."
The Black Ferns sevens team, who compete at the Olympics, have received national contracts for a number of years, but the new deal for 15s players has been seen as pivotal for women's sport in New Zealand.
"It's an exciting time for women's rugby and I'm very proud we've taken the step forward to contract our female 15s players along with our sevens squad," NZR's head of women's rugby Cate Sexton said.
The Black Ferns face Australia in two matches in August, both as curtain-raisers to the first two Rugby Championship tests between the two nations' men's sides, the All Blacks and the Wallabies.
The Black Ferns will also play the United States in Chicago in November, with further matches to be confirmed.
(Reporting by Greg Stutchbury; Editing by Ian Ransom)
