The claims, which have been circulated online, suggest the singer fell pregnant as a young teenager and the baby was raised by her parents as her sister.
In a bid to explain the strange theory, it's claimed that Beyonce is actually seven years older than she claims to be.
Her given date of birth is September 4th, 1981 - meaning she would have been too young to have given birth to Solange, who is 32 and was born in 1986.
However, the theory suggests Beyonce has rolled back the clock on her age and was actually born in 1974 - making her 44.
It would mean Beyonce would have been pregnant at 12 or 13.
The rumour is believed to have been started by an online post from someone who claimed to have spent four years working at Beyonce's Columbia record label.
They shared an in-depth of the unfounded age-change rumours, and claiming Beyonce's cousin confirmed Solange was actually the singer's daughter.
The post stated: "Beyonce's precise birthdate is 9/4/1974.
"I saw a picture of her driver's licence that was on my manager's computer once don't ask how.
"Beyonce and Solange are both teenage mothers, I do not know the paternity of Solange."
The poster goes on to explain where she heard the baby story, writing: "I have heard many crazy things while working at Columbia for four years.
"The Beyonce stuff a close relative of hers told me so I am more prone to believe that..
"Bee's cousin told me Beyonce's daughter is Solange and she said it to me in confidence after the topic of teenage pregnancies came up.
"She is not the sarcastic type either so I doubt she would make something up that is so scandalous about her own cousin."
If the story was true, it would make Beyonce a grandmother as Solange gave birth to a son, Daniel, when she was 18.
The theory has done the rounds online and many believe they've found proof that Beyonce is lying about her age.
It stems from an interview given by actress Gabrielle Union, who claimed that she and Beyonce have been friends "since we were teenagers".
Gabrielle is 46 and was born in 1972 - so it's impossible for her to have been a teenager at the same time as Beyonce if the singer was born in 1981 as she claims.