The actress could be seen standing in front of a beautiful arrangement of pink, purple and blue balloons which hung over the fireplace in her home.
Below stood a huge glitzy 40 decoration, giving the display a very showbiz feel to match Sheridan’s profession.
The star opted for a pair of comfortable blue denim jeans and a smart white coat with black trim.
She happily smiled as she gazed at Billy while holding him in her arms.
Sheridan captioned the post: ‘Thank you all so so much for your beautiful birthday messages! Means the world Xx #40yearsyoung.’
The Gavin and Stacey actress announced the birth of her baby boy in May last year after welcoming him with partner Jamie Horn.
The television star posted a sweet photo of herself holding his tiny hand to her Instagram Stories.
She wrote: ‘Our little man has arrived! We are both completely overwhelmed with love.’
Sheridan confirmed she was expecting in October 2019 while talking to an audience on a cruise ship.
She said: ‘I’ve got a baby on board.’
Meanwhile, this month Sheridan said she wouldn’t rule out reprising her role as Janet in Two Pints of Lager and a Packet of Crisps if the show were to have a Friends-style reunion.
The actress and singer starred in the hit BBC sitcom from 2001 until 2011 as Janet Keogh, the ditzy bakery worker and girlfriend of Jonny (Ralf Little), who loves nothing more than spending nights with her mates at The Archer pub.
‘I mean that would be brilliant, Gavin and Stacey did their special which I thought was brilliant,’ she said. ‘We might end up getting the Two Pints gang back together, I certainly wouldn’t rule that out one day if everyone’s schedules worked out.
‘It was a great part of my life,’ Sheridan enthused. ‘And I guess most people have their university years where they go away and do that but our Two Pints days were like the university years.
‘We all got really close – we were in it for nine years so it was a big part of my life. Definitely don’t rule out that there would be a reunion one day, although it would be weird because we are all so much older.
‘It would be quite funny to see where Janet ended up. She’d probably have about 20 kids or something, still in the pub!’
Got a story?
If you’ve got a celebrity story, video or pictures get in touch with the Metro.co.uk entertainment team by emailing us email@example.com, calling 020 3615 2145 or by visiting our Submit Stuff page – we’d love to hear from you.
MORE : Scarlett Johansson reveals plans to leave Marvel: ‘If you love something, you need to set if free’
MORE : Amanda Holden tells Piers Morgan he’s ‘punching’ as he marks anniversary with wife Celia Walden
Thanks for reading the whole article. If you wish to get an daily update about Sheridan Smith celebrates 40th birthday with baby Billy in sweet photo , click on the bell button to subscribe for the notifications.