Matt also shared his first official couple photo with Keira to his Instagram account.
While Keira wore an all-black ensemble with her toned midriff on display, Matt stood behind her in a blue jumper and smiling at the camera.
Many of Matt's former Love Island co-stars also weighed in on the new relationship in the comments.
"Cuties," season two winner Anna McEvoy commented, while Aaron Shaw simply wrote, "There it isssss."
The couple first went Instagram official when Keira confirmed in a story that she didn't need Bachelor in Paradise to find love.
The blonde beauty, who starred on Richie Strahan's season of The Bachelor in 2016, shared an intimate selfie of herself and Matt cuddling and referred to him as "my love" in the caption.
Keira and Matt were also spotted holding hands and kissing on the streets of Melbourne before Keira's episodes in Paradise had even aired.
Keira and her new man had an awkward run-in with her ex Jarrod Woodgate, who she first met on the first ever season of Bachelor in Paradise.
In a cringe-worthy pair of Instagram Stories, Jarrod revealed to his followers that Keira and Matt were still using his Netflix account and that Matt had privately messaged him to check where they were up to on the documentary series The Last Dance.
"If you think your Netflix account is getting hacked by Russian spies, because it keeps saying 'continue watching' on shows you've never seen... change your password," he wrote.
Jarrod then shared a screenshot of Matt's message along with the caption: "Because it could be the ex you broke up with over a year ago and her boyfriend using your account. WTAF."