Una, 34, melted the hearts of her Instagram followers as she posted the black and white snap of one-year-old Tadhg with the caption, "This smile lights up my world!"
Handsome Tadhg - who looks just like his rugby playing dad, Ben, 31, - wore a star-patterned bandana and a broad grin on his face as he posed for the picture.
Just a few days earlier, Una, who also has four-year-old daughter Aoife with husband Ben, reunited with her bandmates, Mollie King, Frankie Bridge, Vanessa White and Rochelle Humes from The Saturdays at a mutual friend's wedding.
But while she insists that The Saturdays are still very much a band and haven't disbanded, it's her solo album that is taking priority for Una right now.
The songstress revealed last month that she had signed a solo record deal with Decca Records, after quietly working on her debut album over the past two years.
Talking during a Facebook Live chat with The Sun, Una said she was "going back to [her] roots," after starting out as a singer-songwriter when she was in her teens.
"It's been nice to take some time away from the spotlight," she said.
"I've enjoyed being at home and being a mum with the kids, chilling out and gathering my inspiration to put this album together. I've been co-writing and the whole album is original music, mostly songs I've worked on at home with the guitar."