The Australian this morning sealed his transfer from Central Coast Mariners for an undisclosed fee, putting pen to paper on a four-year deal.
The 21-year-old left-back, who was part of an A-League title-winning team last season, will challenge Connor Ogilvie and Jack Sparkes for a first-team spot in the Championship.
Farrell represents John Mousinho’s first signing in more than three weeks after Sammy Silvera arrived on a season-long loan from Middlesbrough at the start of the month.
He also joins Jordan Williams, Josh Murphy, Reuben Swann and Jordan Archer as new faces at Fratton Park ahead of their Championship campaign.
In addition, Zak Swanson last week marked his return to the League One title winners after signing a new one-year deal.