The Widowdating.co.uk website said: "We try to help single widows and widowers in the UK meet someone special. It's not easy to lose a loved one and you can never replace that person, but it can ease some of the pain when you can share your life again with a new friend."
It went on to say: "Widow dating is the one of the largest dating websites for single widows and widowers in the UK."
A woman who joined the site complained that the claims were misleading because she had discovered that most of the members were not widows or widowers.
Widow Dating said the site did not claim that it was exclusively for widows and widowers, pointing out that single, divorced or separated people could also join the site.
It said it had tested the site as both a male and a female member and found that the default search showed members who were predominantly, but not exclusively, widowed.
Upholding the complaint, the ASA said the prominent heading "Widow Dating" and text on the site suggested that members would be widowed people looking to meet other widowed people.
The ASA said: "Because Widow Dating had not supplied evidence that showed that was the case, we concluded that the claims had not been substantiated and were misleading."
It ruled that the ad must not appear again in its current form, adding: "We told Widow Dating not to imply that their dating site was for widows and widowers unless they had evidence that showed all the members of the site had that status."