Swaziland's Princess Simile Dlamini, a niece of King Mswati III, lashed out at actress Amanda du-Pont for the assertion, according to the Swazi Times newspaper.
The comedienne, famous for roles in South African soap operas, affirmed her royal background in an entertainment talk show on eNews Channel Africa (eNCA) over the weekend.
"I am a Swazi princess and people refer to me as 'Nkhosi' back home. They also bow when they greet me," she said.
"Nkhosi" is a term of respect in the Swazi language.
But Princess Simile branded her a liar on photo posting site Instagram.
"Watching Amanda du-Pont lying about being a Swazi princess on eNCA. I've heard about this lie but it's the first time I watched it come out of her mouth -- unbelievable," she was quoted as saying by Swazi Times.
AFP was not able to access the account.
Commentators on Twitter were also quick to react.
"Lol. I really just wonder who in Swaziland refers to Amanda Du Pont as 'Nkhosi' because she's a princess..." tweeted Thuthu Ngcamphalala.
Swaziland has a vast clan of upper and lower nobility and it is sometimes difficult to confirm claims to royalty.
Mswati, Africa's last absolute monarch, has over 20 children and last year took what was thought to be his 14th wife.
His father, Sobhuza II, had 70 wives and 210 children when he died in 1982.