Rihanna won't be hitting the stage in Sweden tonight due to an unknown illness.

The 23-year-old diva, all hooked up with tubes, Tweeted the above photo from her hospital bed ... and the words, "Sorry Malmo!!!"

Live Nation, who is putting on RiRi's Loud Tour, says the singer is "not in condition to conduct the concert."

Rihanna is scheduled to perform on Tuesday in Stockholm, Sweden.