The singer had worked with Flack presenting The X Factor and The Xtra Factor together and was close to her, posting that he thought of her every day.
In an Instagram post, he shared a video of them singing Craig David's song Rendezvous together and wrote about her love of singing.
He captioned it: "Wish I had the full video of this makes me smile every time love how much you loved singing more than me! think of ya everyday happy birthday caz!"
Flack died on 15 February, 2020 from suicide, aged 40.
She had been struggling with her mental health after being charged with assaulting her boyfriend Lewis Burton, something she had denied.
The TV presenter had stood down from presenting the winter series of Love Island as the CPS pursued the case shortly before her death.
Many of Flack's other friends also posted birthday messages sharing how much they missed her.
Iain Stirling, who narrates Love Island which she presented alongside him, posted a photo of them together captioned: "Happy Birthday Caroline x"
Keith Lemon posted a sketch of Flack with the words "Be Kind!" and the caption: "Appy birfday Flack! Miss you x"
Dawn O'Porter posted a photo of Flack and wrote: "Happy birthday you plum."
After Flack's death, her fans popularised #BeKind in recognition of the backlash she had faced over the alleged assault on Burton.
Coroner Mary Hassell, ruling her death as suicide, told the court: "Caroline had fluctuating mental ill health, she had had struggles in the past. She had had difficulties. In spite of the fact she may have led – to some – a charmed life, actually the more famous she got the more some of these difficulties increased – she had to deal with the media in a way most of us don’t.
"It was played out in the national press – and that was incredibly difficult for her. She faced the prospect of not working in the job she loved, losing a great deal."
Watch: Caroline Flack's mum makes emotional speech at mental health awards