The stock later pared those gains but still ended the day 5 percent higher.
On Thursday, Weight Watchers introduced its "Live Fully" campaign with two new ads. Winfrey, a shareholder and the company's most famous customer, touted that she lost 40 pounds since she started the program.
"Weight Watchers is easier than any other program I've ever been on. It's a lifestyle, a way of eating and a way of living that's so freeing. You never feel like you are on a diet and it works," the media guru said in the announcement.
Winfrey and Weight Watchers announced a partnership in October 2015, causing the company's share price to instantly double. Months later, she said she had lost 12 pounds on the diet.
Thursday's announcement didn't say how much Winfrey weighs.
Despite Thursday's gains, the stock has still fallen 51 percent so far this year.