29-year-old Treadwell first joined Ulster back in 2016 after he showed potential with both Ireland U18s and England U20s.
The Irish-qualified second row quickly established himself within the senior squad with his strong lineout calling, high work rate, and his carrying ability.
The Sutton-born man has made 11 caps for Ireland on the international stage, including being a part of the 2022 tour of New Zealand and a member of the 2023 Guinness Six Nations Grand Slam winning side.
A popular member of the squad, he has made over 170 appearances for the province and is a reliable presence in the pack.
On the news, Ulster Rugby’s General Manager, Bryn Cunningham, commented:
“Kieran came to Ulster as a young man with a lot of potential and he developed into an important member of the senior men’s squad.
“Being a key member of our forward pack over the years, he has always brought great athleticism to the game.
“It was a great source of pride to see him called up for Ireland and play in the Six Nations and on the tour of New Zealand in 2022.
“While doing a great job at supporting our young forwards coming through, there’s no doubt everyone will miss his personality around the group.
“It’s fitting that he is returning home to Harlequins for the next chapter in his career, and everyone at Ulster Rugby wishes Kieran and his family all the very best for the future. We look forward to seeing him back ‘home’ in a few years!”
Commenting on the move, Treadwell said:
“I arrived in Belfast eight years ago as a 20-year-old with a chance to represent Ulster and a dream of playing international rugby.
“I never knew when I arrived how much playing for Ulster would mean to me and how much I would love living here.
“I leave as a man who has represented the province over 170 times and fulfilled my ambition of playing international rugby.
“I met and married my wife Beth in Belfast and made a home here that we look forward to returning to in the future.
“I’ve made friends for life here and had so many amazing experiences on and off the field. I want to express my heartfelt thanks to everyone within Ulster Rugby and the wider community for the time I have had here.
“I leave with immense pride and fond memories of my experiences.
“Thank you, Kieran.”