Under Armour and Stephen Curry have mutually agreed to end their 13-year partnership effective immediately, they said on Thursday.

The sudden announcement separates Curry Brand, which makes basketball shoes and apparel, from Under Armour at a time when the sportswear brand is struggling to kickstart sales. As part of the split, Curry will maintain sole ownership of Curry Brand and is free to find another retail partner. Under Armour will release the Curry 13, the final Curry Brand shoe with Under Armour, in February.

"For Under Armour, this moment is about discipline and focus on the core UA brand during a critical stage of our turnaround," Under Armour CEO Kevin Plank said in a statement. "And for Stephen, it's the right moment to let what we created evolve on his terms. We'll always be grateful for what he's brought to the UA team." 

"What Curry Brand stands for, what I stand for and my commitment to that mission will never change, it's only growing stronger," Curry said in a statement. "I'm excited for a future that's focused on aggressive growth with a continued commitment to keep showing up for the next generation." 

Under Armour's split with Curry was announced alongside an expansion of the company's restructuring plan. The company now expects it to cost $255 million, $95 million more than it previously said, including the separation of the Curry brand, additional contract terminations, impairment charges and severance costs. 

Under Armour's basketball business, including the Curry brand, is expected to bring in between $100 million and $120 million in revenue during the current fiscal year, which will end early next year, the company said in a news release. The separation is not expected to have a "significant effect" on Under Armour's financial results or profitability, it said.

Under Armour's breakup with Curry, one of the most successful and widely known basketball players of his generation, comes at a difficult point for the brand. The company has seen a revolving door of top leadership, sales have fallen for eight straight quarters and Plank is trying to it as a premium brand at a time when consumers aren't spending as much on new clothes and shoes.

Plus, the competitive landscape has changed dramatically since Plank founded the business in 1996. These days, even industry leaders like Nike are struggling to hold on to their market share in the face of upstarts like On and Hoka, which have been wooing top athletes and winning over a new generation of customers. To reignite sales, Plank has worked to fix Under Armour's assortment and redefine the brand more clearly in a strategy he said will begin to show up on shelves, stores and social channels this fall and winter season.

Under Armour shares have fallen about 40% this year. Nike and Adidas, which both have partnerships with an array of top NBA players, did not immediately respond to requests for comment.

Curry's partnership with Under Armour had long been a focal point for the brand and presumably would have been a major part of its turnaround strategy. Given the state of Under Armour's business, the abrupt breakup raises questions about what may have driven the split.

Over the last nine years, Curry was mentioned on just about every quarterly earnings call Under Armour held, according to a review of transcripts dating back to 2016. But in its most recent one last week, there was no reference to Curry or the partnership.

During the call, Plank talked about the brand's opportunities in football and intimated it could do more in basketball.

"You can take a category like basketball that's roughly $100 million globally for us all in. And we think to ourself it's incredible for a $5 billion company, that can't exploit that in a bigger, better way," said Plank. "So being underscaled relative to the potential we have and the opportunity that we have to grow it. So we're approaching that positioning as to where do we have the right to play, the right to win, and we think we can just do a little better."

In 2013, Curry shocked the sneaker world by signing with Under Armour over Nike. At the time, Under Armour was a much smaller brand, but Curry was drawn to the company's "underdog mentality."

Curry told CNBC Sport earlier this year he hoped Curry Brand will outlive his playing career and become a brand other athletes embrace — similar to Nike's Jordan Brand. The four-time NBA champion and two-time league MVP is 37 and nearing the end of his basketball playing career, and he has started to focus more on golf and other business activities outside of basketball.

Curry Brand launched in 2020. In 2023, the brand signed a new long-term extension and made Curry the president, housed under Under Armour's banner. As part of that deal, the 11-time NBA All-Star was given 8.8 million Under Armour common shares, valued at $75 million at the time, in addition to other awards and incentives.

Under Armour will continue to develop new UA Basketball products after parting with Curry, according to a statement.

"This move lets two strong teams do what they do best," Plank said in the statement. "Under Armour is focused on product innovation and performance for athletes at every level. Curry Brand gets the independence to determine its own future. That's good for Stephen and good for UA." 

— CNBC's Jessica Golden contributed to this report