Donny Osmond -- I Should Have Used Protection!

Donny Osmond is a little bit ... burnt.

The 55-year-old entertainer took time out from performing with his sister Marie Osmond in Las Vegas ... and unsuccessfully attempted to tan his lily white Mormon skin in Hawaii on Thursday.

After charring his torso to a crisp on the beach ... the Osmond brother continued to enjoy his moment in the sun by wisely covering up with an undershirt.