Mahone shared a picture on Instagram of himself, along with the message: "I've never felt so bad."
A representative for the 17-year-old said Mahone was admitted Thursday with a 104 temperature, extreme inflammation in his throat and severe dehydration.
The teen heartthrob launched into stardom on YouTube. The music video for his single "What About Love" has close to 50 million views.
Mahone won the artist to watch award at this year's MTV Video Music Awards. He's expected to release his debut album next year.
The Associated Press contributed to this report.