Fincham made his surprise announcement on Instagram, explaining that while he is usually open about his life on social media, he chose to keep “this blessing” a secret.
“I’m normally a very open book and share most of my life on my social media. This blessing I chose not to,” he wrote.
“I’m just a normal geeza, who went on a TV show, which throws you straight into the public eye... which I will forever be grateful for.”
The 28-year-old said that he knew he needed “personal time” in order to prepare for the responsibility of becoming a father.
The reality television star described the arrival of his daughter as “the best thing that has ever happened” to him, adding “the instant feeling of love and connection you have with your child is a feeling like no other”.
“I want to thank my family and friends for your continued support, also to my little girl’s mother, who has been totally AMAZING throughout,” he said.
While Fincham did not reveal the identity of his daughter’s mother, he informed his 2.2 million Instagram followers of his daughter’s name: Blossom Fincham.
The former stationary salesman has been inundated with messages of congratulations, including several from fellow Love Island contestants.
“Omg this is amazing,” wrote reality television star Rosie Williams. “Congratulations Jack!!! So happy for you!!! Give her a kiss from me.”
“I’m coming in for a third comment I’m so chuffed for you,” commented Dr Alex George. “Anything you, mum or the baby need mate just let me know.”
Fincham won the 2018 series of Love Island with Dani Dyer, being awarded a joint cash prize of £50,000.
The couple stayed together for around eight months after the end of the programme, announcing their separation in April 2019.