Cleopatra Bernard posted a photo of an ultrasound picture on her Instagram account, writing: “He left us a final gift.”
The Instagram account for the rapper, whose real name is Jahseh Onfroy, later posted the same image with the caption “baby jah #longlivejahseh” with a heart emoji.
It is not known who the mother of the child is.
At the time of his death, the rapper was awaiting trial after pleading not guilty to domestic abuse against his former girlfriend, Geneva Ayala.
In 2016, prosecutors charged him with aggravated battery of a pregnant woman, false imprisonment and witness tampering.
The news comes shortly after police arrested a 22-year-old suspect in connection with the fatal shooting.
Dedrick Devonshay Williams has been charged with first-degree murder without premeditation after XXXTentacion, 20, was shot dead on Monday afternoon.
He was also charged with a probation violation and for not having a valid driver’s licence.
Court records show Williams, who is being held without bond, has been charged previously with several felonies, including grand theft auto, domestic violence, cocaine possession and possession of a firearm by a convicted felon.
It is not clear if he was jailed as a result of these charges, some of which were dropped.
After Williams’ arrest, Broward Sheriff’s Office spokeswoman Veda Coleman-Wright said homicide detectives were seeking additional suspects.
The rapper was attacked by two hooded assailants after leaving a motorcycle dealership in Deerfield Beach, north of Miami.
XXXTentacion’s attorney, David Bogenschutz, said on Tuesday that investigators told him the rapper had visited a bank shortly before the shooting and possibly withdrew cash to buy a motorcycle at Riva Motorsports in Deerfield Beach.
The rapper, whose stage name is pronounced “Ex Ex Ex ten-ta-see-on”, was shot while in his sports car.
XXXTentacion’s album, titled ?, hit number one in the charts and he had a top ten hit with his track Sad!.
A crowd-funding page for Ms Ayala, which includes pictures of her injuries and hospital letters, has raised more than $36,400 (£27,500) to help pay for surgery.
On the page she writes: “It’s crucial I get the surgery otherwise my eyeball will die and the nerve connecting my lip/cheek and teeth will eventually die as well.
“I’d appreciate anything, this is a very strenuous obstacle that has been put into my hands. Also could use money towards my living situation and food. Thank you.”