A source close to the LA-based stylist said she gave birth to a baby boy on Thursday, January 21 in El Lay, reported People magazine.
"Louis is happy and very excited about becoming a dad and he thinks Briana will be an amazing mother," their mutual friend said.
"It was a surprise at first, but he and Briana are very, very close friends and this has brought them even closer."
Tomlinson, 24, is currently taking a break along with his 1D bandmates.
He has not issued a statement regarding his newborn son, but his former stepfather Mark Tomlinson wrote a cryptic message on Twitter, "Is that the patter of tiny feet?"
In July, reports broke out that she's pregnant with his child. He is now romantically linked to "The Originals" actress Danielle Campbell.