Iranian businessman Farhad Moshiri has bought a 49.9% stake in Everton, the Goodison Park club says.
Moshiri, 60, sold his stake in Arsenal to business partner Alisher Usmanov on Friday so he could buy into Everton, who play Chelsea in the FA Cup quarter-finals next month.
"After an exhaustive search, I believe we have found the perfect partner to take the club forward," Everton chairman Bill Kenwright said.
The deal needs Premier League approval.
"I have got to know Farhad well over the last 18 months," Kenwright added. "His football knowledge, financial wherewithal and true blue spirit have convinced me he is the right man."
Providing there are no problems, it will bring to an end Everton's decade-long search for new investment.
Kenwright, who bought Everton from Peter Johnson for £20m on 26 December 1999, said he was prepared to sell as far back as November 2007.
After years of inertia regarding a sale, there has been a growing intent to do a deal in recent months, fuelled by serious interest from a number of parties.
Kenwright has also been suffering from poor health and has rarely attended Everton games this season.
Talks did take place with an American consortium earlier this month, although they did not provide a clue to the eventual outcome.
"Bill Kenwright has taught me what it means to be an Evertonian," Moshiri said.
"There has never been a more level playing field in the Premier League than now."
Everton manager Roberto Martinez has put together an impressive squad, including spending a club record £28m to sign striker Romelu Lukaku from Chelsea.
Silverware continues to elude them. Everton were beaten by Manchester City in the semi-finals of the Capital One Cup last month and remain without a major trophy since they won the FA Cup in 1995.
Chief football writer Phil McNulty
The announcement that Farhad Moshiri is to be Everton's new major shareholder ends a decade-long search for investment by chairman Bill Kenwright.
Moshiri has bought a 49.9% stake in the club and has promised new investment - which indicates there may be even further new arrivals in the Goodison Park boardroom in the months ahead.
Kenwright has consistently stated he would step aside if the right investor could be found and believes he has found the "perfect partner" in the 60-year-old billionaire who sold his 15% share in Arsenal to his business associate Alisher Usmanov for around £200m on Friday.
For Moshiri's part, it means he can now exert the power and influence his money could never buy at Arsenal, where he and Usmanov could not wrest control from majority shareholder Stan Kroenke.
There will be some major items on the new Everton owner's agenda when he settles to his task, namely the next move on whether the club moves ground or considers redeveloping Goodison Park - and also how to keep the club's most coveted players, with top scorer Romelu Lukaku and England pair John Stones and Ross Barkley expected to attract serious interest this summer.
For Everton, however, they will hope Moshiri's arrival will finally provide them with the financial power they have lacked to threaten the elite at the top of the Premier League.