He wrote: “I can’t believe this was 4 years ago today. It’s gone by so quickly. Thanks for all the lovely messages.
“We’re having a lovely anniversary, our first with our beautiful baby girl.”
He signed off the message with a baby emoji and the initials “D&A.”
The Britain’s Got Talent host also shared a black and white photo from his wedding to Astall, his former manager.
Declan Donnelly and Ali Astall with Britain’s Got Talent co-star Ant McPartlin and his ex-wife Lisa Armstrong (Jonathan Brady/PA)
The pair married in 2015, tying the knot at St Michael’s Church in Elswick, Newcastle.
Donnelly and Astall reportedly received wedding messages from the Pope and Prince of Wales, with the presenter’s friend and Saturday Night Takeaway co-host Ant McPartlin as best man.
They welcomed their first child, a daughter called Isla Elizabeth Anne, in September last year.
Declan Donnelly celebrates fourth wedding anniversary with wife Ali Astall