Reid had a queue of suitor clubs from his native Victoria seeking his signature ahead of coming off-contract at the Eagles at the end of next season.
But the 20-year-old has decided to stay at West Coast until the end of 2028 in a massive coup for the wooden-spooners, who won just one game this season.
"I'm incredibly excited to sign on," Reid said in a club statement on Tuesday.
"Since the day I arrived in Perth, the West Coast Eagles members and fans have been so supportive of me and our team, and the future with our young squad is super exciting.
"Playing at Optus Stadium in front of our fans is an unbelievable experience and I love representing the club.
"The support from our coaches and my teammates has been amazing over the last two years.
"And I'm looking forward to continuing to develop my game and helping the team climb back up the ladder.
"I can't wait for the 2026 season and beyond to see what we can achieve together."
West Coast's head of list management and recruiting Matt Clarke said the club was thrilled to re-sign the No.1 pick from the 2023 draft, who is regarded as the best young talent in the competition.
"It's a fantastic commitment to the build that we are undertaking both on and off the field," Clarke said in a statement.
"Harley is a very important part of our future and has always displayed a great connection to our football club throughout the process.
"He embodies everything we want in a West Coast Eagle - skill, determination and a fierce competitive spirit.
"The future with Harley and his family on board with the West Coast Eagles is something that we feel will be very special."
Rival clubs including Hawthorn, Collingwood, St Kilda and Essendon had pitched to lure the brilliant midfielder. The Saints' offer was reportedly worth $25 million over 11 years.
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