By Svea Herbst-Bayliss

NEW YORK, March 17 (Reuters) - Activist investor Jana Partners wants theme park operator Six Flags Entertainment FUN.N to explore a sale and immediately appoint a new head of its board of directors, according to a letter reviewed by Reuters.

The call for change comes just months after North America's largest regional amusement resort operator hired a new chief executive and less than a week after it appointed National Football League star Travis Kelce as brand ambassador.

In a letter addressed to Six Flags' board, Jana cites concerns about the board's ability to "deliver" for shareholders and calls on the company to engage with buyers.

"It is now in the best interest of shareholders for the company to reverse course and engage with known buyer interest in Six Flags," Jana Managing Partner Scott Ostfeld wrote.

The firm, one of the industry's busiest activist investors, also wrote that board leadership changes are needed and the company should appoint a new chair. Marilyn Spiegel was named chair in January and has been a director since 2023.

The company did not immediately respond to a request for comment.

In October, Jana unveiled its stake in Six Flags along with its partnership with Kelce, the Kansas City Chiefs tight end who is engaged to singer Taylor Swift. At the time, the hedge fund, which disclosed a roughly 9% economic stake, said it wanted Six Flags to improve operations, revitalize its marketing and improve the customer experience at its parks.

It also suggested that the company could be an attractive acquisition target.

Investors pushed Six Flags' stock up nearly 20% on the news. Before Jana's stake was revealed, the company's stock price had tumbled some 50% year-to-date as rainy weather kept visitors out of the parks and weighed on returns.

Since then an earnings miss and worries about business performance have pushed shares lower, leaving the company with a market value of $1.7 billion. The stock price closed at $16.39 on Monday.

While Jana has publicly expressed support for new CEO John Reilly, it said in the letter that it wants to see a new board chair after months of private engagement raised concerns about the group's effectiveness.

"We have witnessed an alarming pattern of board dysfunction and disjointed decision-making that has become impossible to ignore," the letter said.

In the letter, Jana cites a string of shortcomings at the board level including how the directors sat on the announcement of Reilly's appointment as chief executive in November for days while the company was experiencing a crisis of confidence.

It also criticized the board's approval to reaffirm financial guidance in September to calm nervous investors only to then slash guidance weeks later.

In February, Reilly had said that while 2025 results had come in short of the company's expectations, "the work completed over the past year has strengthened the foundation of our enterprise."

He said the company improved park infrastructure, added new attractions to parks, upgraded technology and enhanced food and beverage offerings. He also said the company's efforts are sure to "restore profitable growth that is sustainable over time."

Jana is not the first activist investor to push Six Flags for changes. In October, only days before Jana's position was unveiled, the company added an executive from activist hedge fund Sachem Head Capital Management, which owns roughly 5% of the company, to the board.

(Reporting by Svea Herbst-Bayliss; Editing by Jacqueline Wong)

((svea.herbst@thomsonreuters.com; +617 233 2138; Reuters Messaging: svea.herbst.thomsonreuters.com@reuters.net))