Kolisi, who launched his professional rugby career in Cape Town, is returning to where it all began. With 118 caps for the Stormers to date, his experience and leadership are expected to play a key role in mentoring younger players.
The move will also allow Kolisi to be closer to his family while contributing to the team that launched his career.
Kolisi made his Stormers debut in 2012 and was appointed captain in 2017. A year later, he was named captain of the Springboks, lifting the Rugby World Cup in 2019 and again in 2023 and cementing his status as one of South Africa's most iconic rugby figures.
Stormers Rugby CEO Johan le Roux was delighted to welcome Kolisi back.
"Siya is a national icon who has such a strong connection to our team and our supporters. He is also a family man who wants to be there for his children as much as possible.
"These are values that resonate with our club and given the significance of our heritage in defining where we want to go, we are very happy to welcome Siya back to hopefully see out his career in the jersey he wore when it started.
Kolisi said that returning to Cape Town is a significant moment for him and his family.
"I have such incredible memories of my time at the Stormers, the way I was embraced by the fans in my formative years has never left me.
"My family is in Cape Town and it is important to me to be close to them. This move gives me the chance to do that while also giving back to the team and fans who made such a big contribution to the player and person I am today.
"I am looking forward to contributing in any way I can to the future success and happiness of this special club and its incredible supporters," he said.
Director of Rugby John Dobson said that Kolisi will add value to the squad in ways that no other player could.
"We have got an outstanding crop of loose forwards in our squad and a number of very strong young leaders, with the last three Junior Springbok captains all on our books.
"Siya will be able to help develop these players and mentor them in a way that nobody else could, given what he has achieved and what he represents to all South Africans and especially our fans.
"We are very aware of the load that he has carried professionally and personally over the last few years, but what is so encouraging is the way that he has bought into helping us reach the goals we have set for ourselves in developing our squad as we build towards Project 2029," he said.
The Sharks paid tribute to Kolisi's wider impact at the franchise, highlighting the leadership and standards he leaves behind as he prepares for the next chapter of his career.
"Siya's contribution to The Sharks goes far beyond matchday," said Sharks CEO Shaun Bryans. "He brought a level of leadership, humility and perspective that lifted everyone around him. While it is never easy to say goodbye to a player of his calibre, we fully respect his decision to put his family first. Importantly, Siya has helped set standards that will remain long after he moves on, and that creates real opportunity for the next group of leaders to emerge."