Few details were available, but media reports were enough to send shares in Weight Watchers up around 16%.
The share price  has tumbled 85% this year on losses and criticism that it has been too slow to react to competition and new health trends.
The New York-based firm, which could be valued at about $2bn (Â£1.3bn), saw profit fall 52% last year.
The hedge fund has reportedly bought Weight Watchers loans, due for repayment in April next year, and is now talking to potential partners about a possible joint bid.
The company is still the world's leading weight loss brand, but fewer people have been subscribing to classes, with mobile apps and activity monitors a contributing cause.
In the US, where the company has the majority of its subscribers, the number of people signing up has fallen by more than 20%.
Weight Watchers International is reportedly a takeover target for an unnamed activist hedge fund.