The 23-year-old joins the Power in return for a fifth round pick (No. 88 overall) at the 2012 NAB AFL Draft, which Port Adelaide has accepted from Melbourne in exchange for David Rodan.
Port Adelaide general manager football Peter Rohde said Stevenson would be an astute pick-up.
"Lewis has often been really close to selection in what is a very good West Coast line-up and was an emergency on several occasions in 2012, including in their semi-final," Rohde said.
"While he couldn’t break into the AFL team, he has done everything right and has been very impressive with his WAFL side Claremont, where he was part of their back-to-back premierships in the past two seasons.
"His 2012 form was possibly the best of his career and he comes very highly recommended by football people in Western Australia."
Trade tracker
Stevenson hasn't played a game of AFL football for two years, but his WAFL coach Marc Webb says it's through no fault of his own.
Drafted to West Coast in the 2007 NAB AFL Rookie Draft, Stevenson played 10 games after being elevated to the senior list in 2010 as the Eagles languished on the bottom of the ladder.
As the side rebounded and key players returned from injury, he slipped down the pecking order behind the likes of Beau Waters and Shannon Hurn.
But he starred in the WAFL and played a key role in securing back-to-back premierships for Claremont.
Webb told AFL.com.au Stevenson's lack of AFL football was an unfair portrayal of his true talent and described his move to the Power was a "great trade".
"'Lewie's' been probably one of our key defenders this year, he was in our best six to eight players all year in his defensive work and ability to run off, and his skills are excellent," Webb said.
"With guys like Beau Waters, Shannon Hurn and Jacob Brennan, it meant he was down the pecking order and it really affected his ability to play senior footy.
"He was playing really good football for us and just couldn't get that opportunity ... West Coast had a glut of those players.
"That'll be a great trade for Port Adelaide because personality-wise he's a great guy, very coachable, works his heart out; that'd be great for him to get an opportunity."
Stevenson finished inside Claremont's top-10 at the best and fairest despite missing a handful of games as West Coast's travelling emergency.
In round 23 he narrowly missed out playing against Hawthorn at the MCG and caught a late-night flight back to Perth, touching down at around 2am.
He woke up, drove to Claremont Oval and collected 20 disposals off half-back to be named in the Tigers' best.
Trade Machine
Webb said the 188cm defender's versatility set him apart from many others; able to play on fast small forwards, larger targets and even push up onto a wing.
"His ability to play on guys like Des Headland, strong forwards that he kept out of the game and at other times to be our key running back was great as well," he said.
"He's still a decent sized guy and can play on mid-ranged guys, but because of his pace he can handle smaller, quicker players as well.
"The range of player he can play on was really important for us and I think, given the opportunity, he's got the ability to come in at AFL level ... he far and away credited himself in the WAFL system.
"At times we needed a creative player on the wing and he played that role really well for us too, it'll be a big loss for us but I think it'd be a good pick up for Port Adelaide because he's a ready made player."
Harry Thring is a reporter for AFL Media. Follow him on Twitter: @AFL_Harry.
