The Irish TV cook, 25, joins MasterChef stalwart John Torode for the second series of the CBBC show.
Skehan developed a passion for food from a young age and has worked alongside some of Ireland's top food photographers and stylists. He has been described by some critics as "Ireland's answer to Jamie Oliver".
He runs a popular food blog on donalskehan.com and released the book Good Mood Food in 2009.
"I'm hugely excited to be joining the MasterChef team as a new Junior MasterChef judge," said Skehan.
"Working with John Torode is incredible - his knowledge of food is fantastic and he's been really supportive and welcomed me into the MasterChef family.
"The series promises to be really inspirational. The contestants are incredibly accomplished - sometimes I even found myself taking notes! We're about to discover some of the most talented young chefs in the UK."
MasterChef creative director Karen Ross said: "Donal's energy, enthusiasm, humour and absolute passion for what he cooks and eats made him a natural choice for our Junior Judge.
"The fact that he and John Torode also hit it off immediately with a shared love of food and an instant cheeky banter is a bonus. We are extremely excited to have him onboard."