Oprah Winfrey and Weight Watchers International are teaming up in an attempt to transform the dieting company into a broader health and fitness brand.
The announcement of the arrangement has sent the company's shares up almost 90%.
The talk-show host and media magnate will join Weight Watchers' board and take a 10% stake in the company, with options to buy more, the two parties said.
Shares of Weight Watchers nearly doubled in early trading, cutting some of their recent losses to hit their highest level since February.
More than 35m shares changed hands on US exchanges, the busiest day in the company's history.
Ms Winfrey will help broaden Weight Watchers' reach and appeal and the company plans to feature her in its marketing, Chief Executive Jim Chambers said.
The media star, who for years has struggled publicly with her weight, will receive options to buy another 5% of the company.
Besides being a board member and an adviser, she has also joined the weight-loss programme and will document her experience.
"Weight Watchers has given me the tools to begin to make the lasting shift that I and so many of us who are struggling with weight have longed for," she said.
Ms Winfrey, 61, was regarded as America's most influential celebrity for many years thanks to her daily talk show, magazine, book club and production company Harpo.
Although her influence has slipped since she ended 'The Oprah Winfrey Show' in 2011 after 25 years to launch the Oprah Winfrey Network on cable TV, she remains one of the richest and most powerful celebrities in the United States. Forbes magazine has estimated her net worth at $3bn (€2.65m).
"The good thing about picking Oprah Winfrey is that although she is very much a really strong media personality, lots of people still see her as being a very ordinary person, a person that has struggled with things like weight loss," said Neil Saunders, CEO of retail research firm Conlumino.
