The singer and Take That star posted a picture of himself with his wife Dawn from more than 20 years ago on Instagram along with a tribute.
He wrote: “Happy wedding anniversary Mrs. B. 18 fabulous years together.
“Stronger than we’ve ever been. Here’s to the rest of our lives.”
Barlow, 46, added that the picture of them was taken in 1995, before they were married, and he used the hashtag #foreverlove, referring to one of his hit singles.
The pop star married Dawn in 2000 after being in a relationship for several years.
The pair had met earlier in his Take That career when she was a backing dancer for the band.
The couple have three children together, Daisy, Daniel and Emily. In 2012 their fourth child Poppy was delivered stillborn.