Koroisau was widely expected to sign with the Bulldogs, having also received interest from the Warriors.
However the Panthers had a long-standing offer on the table which Koroisau elected to take up.
The premiership-winning rake played with the Panthers in 2015 before following former coach Trent Barrett to Manly, where he has plied his trade for the last four seasons.
He will now battle it out with Mitch Kenny for the starting hooking spot in the Panthers side next year.
“I’m thrilled that we’ve been able to bring Api back to the club,” Panthers coach Ivan Cleary said.
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“I had a high opinion of Api when I coached him previously and his game has only continued to develop in recent years.
“He’s a man of good character and will provide valuable experience and leadership for our young squad.”