The Scottish meteorologist was broadcasting from West Wittering beach in West Sussex when a dog called Connie - belonging to a BBC producer - crept into shot and began to urinate, with the shallow water lapping at its heels.
BBC Breakfast presenter Louise Minchin spotted the blooper-in-the-making, and warned: "Oh Carol, don't look behind you!"
Co-host Charlie Stayt added: "Don't turn round!"
Spotting the dog as it sauntered off, Kirkwood replied: "Upstaged by a dog, it's the story of my life."
The BBC Breakfast Twitter feed later carried an image of the dog with the message: "Someone's looking a bit guilty..."
It also added: "Soooo... that was awkward. Rest assured: our sources tell us it was a number one."