Fisher Investments — the giant investment fund headed by billionaire Ken Fisher — has sold a minority stake in the firm that values it at nearly $13 billion.

Under the deal, Boston-based private equity firm Advent International and the Abu Dhabi Investment Authority will invest at least $2.5 billion in Fisher Investments, a closely held firm that values itself at $12.75 billion, the company said in a statement on Monday.

The firm’s founder and executive chairman, who is worth an estimated $5 billion, said the agreement would guarantee the firm’s “independence” if he ever decides to stand down.

“While my health is excellent, this transaction with an atypically long holding period for a private equity transaction will ensure FIs long-term private independence and culture should anything untoward happen to me,” Fisher said in a statement.

It is the first time in the history of the company, run by 73-year-old Fisher since 1979, that it has welcomed any outside investment, with shareholders being limited to the Fisher family and employees.

Fisher — who in addition to running the firm also is a regular financial columnist for The Post — will retain a 70% stake in the business, The Wall Street Journal reported.  

Fisher Investments says it handles roughly $275 billion of investments for more than 150,000 clients worldwide.

Reports that a minority stake in Fisher Investments was up for grabs first surfaced in January of this year.

The company switched its headquarters from Washington state to Texas last year over hikes in capital gains taxes.

Fisher served as CEO for 37 years before he handed the reins over to Damian Ornani in 2016.

Advent, founded in 1984, snapped up Nuvei, a Canadian payments technology company that has received financial backing from actor Ryan Reynolds, in April in a deal worth close to $6 billion.