In another statement signing, the Chiefs announced on Monday that To'o will join great mate Jarome Luai in Port Moresby in 2028 for PNG's inaugural NRL season.
To'o was due to enter the final 12 months of his Panthers contract on November 1 but was given a short window to open talks with the Chiefs and strike a deal ahead of time.
Penrith were reluctant to allow negotiations to drag on as September approached with the club firmly in the race for the minor premiership and for a fifth title in six seasons.
The winger was given a week to speak with the Chiefs and informed Penrith powerbrokers last Wednesday - a little more than 48 hours before their loss to the Warriors in Auckland - that he would be leaving for PNG in 2028.
"The club has maintained a strong position that this cohort of players has absolutely earned the right to choose what is the next step in their rugby league journey," said Penrith chief executive Matt Cameron.
"We are so proud of what Brian has been able to do for our community and fully respect the decision he has made.
"It's an exciting period for the game as the NRL continues to grow, and we believe a strong competition is ultimately good for rugby league."
While the loss of the 27-year-old To'o is a blow for the Panthers on the field, the 17-time State of Origin winger's exit gives the club clarity on their retention priorities heading into next season.
Penrith have a further eight players - including Nathan Cleary, Isaah Yeo and Moses Leota - entering the final 12 months of their deals in November and To'o's departure gives the Panthers additional room in their salary cap.
To'o didn't travel to Port Moresby to see the club's resort-style accommodation but is expected to see his future home when he heads to PNG as part of Samoa's World Cup squad later this year.
The influence of Luai, who was best man at To'o's wedding, is said to have been significant in convincing the winger to become the Chiefs' sixth official signing.
Roosters utility Connor Watson, PNG Test winger Alex Johnston, code-hopping three-quarter Zac Lomax and English front-rower Matty Lees are the other four players to have confirmed their intention to join the Chiefs in 2028.
Ex-Roosters centre Joey Manu visited Port Moresby last month but has since returned to France where he currently plays for Top 14 rugby union side Racing 92.
"Brian understands family, sacrifice, humility and togetherness," said Chiefs general manager Michael Chammas.
"Those are values shared across Papua New Guinea and the Pacific.
"To secure a player of Brian's quality, character and cultural strength is a huge moment for the Chiefs."
