The explosive former Tonga captain is still only 32 but will finish his impressive career at the end of this season.
A World Cup winner with New Zealand in 2008, he also lifted the NRL title with Melbourne Storm four years later before joining Penrith Panthers and then endearing himself to the Hull faithful.
A destructive carrier, Manu has made more than 100 appearances for FC and played in both their famous Challenge Cup final wins at Wembley in 2016 and 2017.
“It was an extremely hard decision to make because I have had an enjoyable career, especially my time here at Hull,” said Manu, who returns from a calf strain to face Huddersfield Giants tomorrow night.
“But I feel that the time is right to hang up the boots. When you give your all on the field every week it certainly takes its toll on your body and I have been feeling the effects of it on the training field this season.
“It’s been an amazing journey and I feel very lucky that I am able to finish up here at Hull FC.
“We’re still in with a chance of making the play-offs and potentially going on towards the Grand Final, so that is the aim and I will give my absolute all to help the team get there.”
Hull coach Lee Radford labelled Manu as “one of the best signings the club has made in the modern era” and expressed how “easy” he is to coach.
Meanwhile, they will be without in-form winger Ratu Naulago tomorrow owing to a “minor knock”.
Jack Logan comes in with Kieran Buchanan replacing Jack Brown while Giants could give a debut to Halifax loanee Chester Butler and Paul Clough returns. Castleford have Liam Watts back for their game at St Helens.