The two-year deal ties the 24-year old to the Red V until the end of the 2022 NRL season.
Faamausili has played 28 NRL games after making his debut in 2018, including a four-game stint at the New Zealand Warriors this year.
Dragons recruitment manager Ian Millward was thrilled to announce the signing of the Auckland native.
“Poasa is a highly talented, emerging front-rower that we are excited to secure for the next two seasons,” Millward told the club website.
“Incoming head coach Anthony Griffin was keen to get Poasa on board and saw him as a point of difference to our forward pack, with his genuine leg speed and physicality.
“Given his experiences in the Sydney Roosters’ system, we are confident there is plenty of improvement in Poasa. We must also thank the Roosters for their cooperation with Poasa’s signing.”
Faamausili was released from the Roosters on the same day that Shane Flanagan was, who has joined Canterbury on a three-year deal.
The Dragons now have four spots remaining on their 2021 roster.