Moa, 26, has signed a two-year deal with the Roosters, having opted to return Down Under due to family reasons.
The Tongan said: “I have had a really special time with Hull and enjoyed every minute of it, not only around the club and with the team but meeting so many people around the city too, so I’m thankful for that.
“With my family moving back to Australia last year it was a tough call to stay in Hull for another year, but it was one I was keen to do.
“Now though the time feels right to go back, I need to be closer to my partner and around my family so it’s a move I make with the best wishes and with fond memories of the club.”
It means Moa has played his last game for the Airlie Birds, having already been ruled out for the rest of the season with an arm injury, barring a lengthy run in the play-offs.
Head Coach Peter Gentle said: “He is a popular player and a big part of the club, on and off the field, so he’ll be missed I’m sure.
“We wish him all the very best in his future career with Sydney Roosters.”
Moa, who joined the club from Cronulla four years ago, has scored seven tries in 78 appearances for Hull.