Walsh was a member of the Power's coaching panel between 1999 and 2008, and he will return to Alberton to work as midfield coach under Ken Hinkley.
Port Adelaide director of coaching and strategy Alan Richardson said the club was delighted to secure Walsh, who is highly regarded at the Power and remains friends with many of the senior players.
“To have such an experienced and well-credentialed coach like Phil return to our club is very exciting,” Richardson said.
“Having spent the last five seasons at such a great club like West Coast, Phil will bring a wealth of experience and fresh ideas to our coaching panel.
“He played such a vital role in our club’s 2004 premiership win and also the 2007 finals campaign. We see him as a perfect fit.
"Players like Kane Cornes and Dom Cassisi have kept in regular contact with him."
Walsh announced he was returning to South Australia early in September, with assistant coach David Teague also departing the Eagles.
On Tuesday night the Power announced Rehn had parted ways with the club after two seasons as midfield coach.
Richardson said it was a difficult decision not to extend Rehn’s contract.
"Shaun has played an important role in improving our midfield group over the last two seasons and by extension has been valuable in our club’s climb back up the ladder this year," Richardson said.