An assistant to dual Hawthorn premiership coach Alastair Clarkson since 2010, Simpson has been viewed as a senior coach in waiting and impressed the Eagles in his interviews this week.
A two time premiership player with North Melbourne, he played 306 games with the Kangaroos and stepped straight into coaching when he retired at the end of the 2009 season.
The 37-year-old will take over from John Worsfold, who stood down at the end of the 2013 season after the club fell from premiership favouritism to a finish of 13th, managing just nine wins.
Simpson said he was looking forward to the opportunity of coaching in his own right.
“I am grateful for this opportunity and I look forward to moving across to Perth with my family and settling in to the challenge ahead,” he said.
“I have always admired West Coast from afar and I believe the list has an exciting mix of senior and emerging young players.
“I would like to acknowledge both Hawthorn and North Melbourne for the grounding they have given to me in this game.
“Hawthorn has been fantastic for me in the last four years and it was certainly a great experience to be a part of the coaching group as the club achieved the ultimate success last Saturday.
“In particular, I’d like to thank Alastair Clarkson and Chris Fagan for the wonderful education and assistance that they have provided to me.
“Being so closely involved from a coaching perspective has provided me with a sense of the hard work which has to be done to achieve a premiership. I obviously enjoyed that feeling as a player, but it is different as a coach.
“I will move across to Perth as soon as possible and I look forward to meeting the players and working with them from the start of pre-season.”
Eagles ruckman Nic Naitanui Tweeted on Thursday evening: "Welcome on board Adam Simpson," then followed a few minutes later with: "And yes I got the text this time haha" in reference to his late notice of Worsfold's leaving.
The original pool of 14 candidates had been whittled down this week, with the Eagles forced to wait for the Dockers and Hawks to finish their campaigns before formal interviews started.
It is understood highly rated assistant Scott Burns was told earlier in the week he was no longer in the running, with a decision left between the final two.
Sumich pushed close to the senior position after a two-year period under Ross Lyon at Fremantle, where his stocks appear to have been boosted significantly.
Eagles chairman Alan Cransberg thanked the unsuccessful candidates.
“I am sure they will continue to add enormous value in assistant roles or any future senior coaching roles,” Cransberg said.
“There were obviously some highly-credentialed coaches who applied for the position and we were delighted with the quality of the applicants.
“It was a tough job for the selection panel and the board to arrive at this unanimous decision, but we believe Adam is best equipped to take the club forward.”
With the Gillette Trade and Free Agency Period set to launch on Friday, the Eagles needed to appoint their senior coach to contribute to list management decisions.
It is believed midfielder Daniel Kerr, who is contracted for 2014, won't make a call on his playing future until he has spoken to the new coach.
