The 26-year-old posted an emotional statement on his Instagram account yesterday, after reports surfaced he had signed with English club Wasps on Wednesday.
What is reportedly a two-year deal worth $NZ1million will rule the 16-test All Black out of the 2019 World Cup.
"It’s with a heavy heart that I announce that my family and I will be leaving NZ at the end of the 2018 season," he posted.
"It was one of the hardest decisions I’ve ever had to make but is one I’m at peace with.
"I have loved every minute of being a professional footballer here in NZ. I’ve made life long friends and memories that I’ll cherish forever."
Sopoaga thanked the Highlanders and All Blacks, as well as his two provincial teams, Wellington and Southland, alongside fans, team-mates, coaches, family and friends.
The Otago Daily Times was unable to reach the Highlanders for comment.
Sopoaga was just 20 when he made his debut for the Highlanders, against the Hurricanes in 2011.
Over time, he became a key player and was influential as the team claimed its first Super Rugby title in 2015. He played 74 matches for the Highlanders.
He made his All Black debut against South Africa at Ellis Park in 2015, in a match leading in to the World Cup.
While left out of the Cup team, he returned the following year to play five tests off the bench. He then made a further 10 test appearances this year, although only one of them was a start.
In his post he said he was ready for "one last roll of the dice" in 2018.