The Night Manager star will take over from the fourth season's Emma Corrin.
Debicki joins Jonathan Pryce and Imelda Staunton for the final two seasons of the royal drama, which is expected to cover the 1990s and early 2000s.
The fourth season is expected to be released this autumn.
"Princess Diana's spirit, her words and her actions live in the hearts of so many," Debicki said in a statement posted by The Crown's official Twitter account.
