"I wanted a change and to switch back to union with the Chiefs," said Fisiiahi.
"I have really enjoyed the league environment but I have rekindled my interest in rugby and want to try a different environment. I'm looking forward to a new chapter in my sporting career and a fresh challenge.
"I have been impressed by what I have seen at the Chiefs and I believe it will be a good fit for me."
Chiefs coach Dave Rennie is looking forward to having the former Wesley First XV product in his side, saying: "Glen is a great addition to our group. In the mould of Asaeli Tikoirotuma, he is electric, has a strong skill set and is an outstanding support player. The opportunity to roam on both sides of the park in our game will suit him."
Fisiiahi is the second Warriors player to make the code switch in the last month, with former centre-winger Ngani Laumape having signed on with the Hurricanes for next season.
The former Tongan international was tipped for a big NRL career after he scored 22 tries in 29 games for the Junior Warriors and helped them to the club's maiden NYC premiership in 2010.
"There has never been a question about Glen's ability. He's a hugely exciting player when he's fit," said Warriors coach Andrew McFadden.
"We've seen glimpses of what he can do, like the four tries he scored against the Tigers in Wellington last year.
"It's been frustrating for him, though, because while the potential was obviously there he just hasn't been able to get on the field often enough and he has now decided he wants to have a shot at rugby union instead.
"Glen is disappointed rugby league hasn't worked out the way he'd hoped and so are we. We'd all been hoping he would come through his setbacks but it wasn't to be. We wish him all the best in rugby union and we'll certainly be watching his progress with interest."
Meanwhile, Warriors front-rower Sam Rapira faces a one-match ban for a grade one dangerous throw on former teammate Lewis Brown in Saturday night's defeat to the Panthers.
Panthers forward Regan Campbell-Gillard has been charged for the grade two careless high tackle that saw Rapira depart play with concussion after just 18 minutes, but with an early guilty plea will be free to play this weekend.
Fullback Sam Tomkins was also charged with grade one dangerous contact also on Brown, but will be free to play next round with an early guilty plea.
