The former One Direction singer took to Twitter to reveal the couple's newborn daughter is ''healthy and beautiful'', and he feels ''proud'' to call her his child.
Zayn wrote: ''Our baby girl is here, healthy & beautiful to try put into words how i am feeling right now would be an impossible task. The love i feel for this tiny human is beyond my understanding.Grateful to know her, proud to call her mine, & thankful for the life we will have together x (sic)''
Gigi admitted she is ''so in love'' with the little one.
She wrote on Instagram: ''Our girl joined us earth-side this weekend and she's already changed our world. So in love (sic)''
The pair - who have been dating on and off since 2015 - both shared black and white photos of their daughter's tiny hand.
Several stars congratulated the couple on their new arrival, including Emily Ratajkowski.
She wrote: ''Congratulations Gigi & Zayn!! (sic)''
Hayley Bieber posted: ''Welcome babygirl! So happy for you guys. (sic)''
Olivia Culpo shared: ''Ahhhh!!!!! HUGE congrats!!!!! I can't wait to meet this angel (sic)''
Gigi first confirmed her pregnancy in April, after ''family sources'' first revealed the news.
She said at the time: ''Obviously, we wished we could have announced it on our own terms but we're very excited and happy and grateful for everyone's well wishes and support. Especially during this time, it's a nice silver lining to be able to be home and be together and really experience it day by day.''
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