Kate made the pregnancy reveal on Instagram on Sunday, as she unbuttoned her coat to reveal a blossoming baby bump in a short clip.
The former “The Only Way Is Essex” star captioned the post: “We’ve been praying for you.”
The couple are parents to son Cree, 2, and Kate is also stepmother to Rio’s children Lorenz, 15, Tate, 13, and Tia, 11 – who he has with his late wife Rebecca Ellison. She passed away from breast cancer in 2015.
Rio also shared the video to Instagram with a number five in a red love heart and a number of stars congratulated the pair on the joyful news.
Former “Love Island” contestant Montana Brown said: “Stop! This is amazing! Congratulations!”
Kate’s former “The Only Way Is Essex” co-star Georgia Kousoulou wrote: “So, so happy for you."
Meanwhile, Kate said last year that she had suffered a heartbreaking miscarriage.
She said: “That walk to the hospital felt like the longest walk ever.
“The minute I looked at the lady, she didn’t even have to say anything, I just looked at her face.
“In comparison to when I had Cree, the facial expression was very different – they’re happy saying, ‘Oh lovely, all’s okay’. And then the lady just didn’t say anything. It was just awful.
“I just burst into tears, she hadn’t even said anything and Rio was like, ‘Calm down’ and I was like, ‘I know, I know, I know’ and they said there’s no heartbeat.”