Long standing Sports Direct chief executive Dave Forsey has resigned from the retailer, with Mike Ashley replacing him.
The firm said on Friday that Mr Ashley, previously executive deputy chairman, was appointed with immediate effect following Mr Forsey's resignation on Thursday.
Mr Forsey, who has been with the retailer for 32 years, said: "I have given my entire working life to the company and in return the company has given me amazing opportunities and experiences. I wish everyone at Sports Direct well in the future."
Sports Direct added that Mr Ashley will continue to be supported by the existing executive management team, and Mr Forsey has agreed to facilitate a "smooth handover of his responsibilities".
Mr Ashley said: "I feel like I have lost my right arm, but I do hope to have the opportunity to work with Dave again in the future."
The news comes after a gruelling year for the business, which has seen Sport Direct come under fire for working conditions at its Shirebrook factory and its corporate governance practices.
The firm also announced that Karen Byers has been promoted to the role of global head of operations and Sean Nevitt has been promoted to the role of global head of commercial, both with immediate effect.
Chairman Keith Hellawell said: "I would like to thank Dave for his significant contribution over the last 32 years and during a period of significant growth and change at the company. On behalf of all the people at Sports Direct I wish him well in his future endeavours."
Mr Ashley has been lambasted for using zero-hour contracts, presiding over "Victorian" working practices and was grilled by MPs in June, where he admitted that some warehouse staff were paid below the national minimum wage.
Earlier this week, Sports Direct said it is to undertake an independent review of working practices and corporate governance, following concerns raised by shareholders.
Mr Ashley said in a TV interview earlier this week that things "need fixing" at the firm.
He said: "We definitely had some things we had to fix, and hopefully in the 90 days we've fixed a lot of those things, and there's still, as I said to them at the time, a lot of stuff ongoing that will always remain and need fixing.
"I've said sorry, I've said I'm going to fix it, and I will."