The 24-year-old took to his Twitter profile this afternoon to share the exciting news, with him gushing over the "pretty amazing" little one.
He posted: "I'm pleased to say my baby son was born yesterday :) He is healthy and pretty amazing :) I'm very happy!!"
Related articles
Before he later added: "Daddy daddy cool," with three smiley faces.
Fans have been quick to congratulate the What Makes You Beautiful hitmaker - who is currently on a hiatus from 1D duties - via social media.
One person posted: "Congrats man!! I bet you're going to be a great father x @Louis_Tomlinson," as another added: "Although I'm a bit speechless right now, Congrats @Louis_Tomlinson , you will make an incredible father."
The hashtag 'Congratulations Louis' has been trending since the news emerged online, with his bandmate Harry Styles also saying: "Flowers all round."
It was first revealed that Doncaster-born Louis was becoming a father in July and the due date was believed to be this month.
At the time a friend told the Mail Online: "Although they're not romantically involved, Louis thinks Briana is a great woman and will be a fabulous mother."
"Louis has been helping Briana financially and is prepared to get stuck in with all the tasks that caring for a baby demands," they added.
Doncaster-born Louis has relocated to Los Angeles to be by Briana's side throughout the pregnancy.
Media mogul Simon Cowell - who has witnessed the band's rise to fame - admitted that he's more than willing to be the baby's godfather.
He told the Mirror: “I’m godfather to a lot of kids now. I would if he asked. We’ll have to wait and see.”