Stars including Sarah Silverman and Isla Fisher commented on her post, wishing her well.
Kate Beckinsale has been admitted to hospital with a ruptured ovarian cyst.
The actress, 45, shared a picture of herself in a hospital bed on Instagram with a breathing tube in her nose and a pained expression on her face.
She wrote: “Turns out a ruptured ovarian cyst really hurts and morphine makes me cry.
“So thankful to everyone who looked after me #wobbly.”
Another picture shared with her 2.5 million followers showed her lying in the hospital bed.
The Underworld star shared no more information, but she responded to her hairstylist Peter Savic, who asked when it happened: “Today!! Thank you darling love you.”
A number of stars shared their concern, with comic Sarah Silverman – a close friend and ex of Beckinsale’s former partner Michael Sheen – wrote simply “OMG!!!!”.
Actress Isla Fisher asked: “Are you okay? Sending you so much love?”
Singer Natalie Imbruglia said: “Oh no… wishing you a speedy recovery.”
A representative for Beckinsale has been contacted for comment.