Sonny Davy, who turns two in August, has a 7cm round capillary hemangioma (abnormal overgrowth) on the left side of his face, which is often swollen and protruding.
Mum Lucy Denny, 25, said: "Sonny's birthmark attracts a lot of unwanted attention. A lot of people stare at him when we are out, and people make rude comments. It is unbelievable how cruel some people can be.
"One person said 'I'm glad my baby hasn't got that on his face."
The 25-year-old is worried the birthmark will make Sonny's life difficult once he gets to school as it could make him a target for bullies
Denny, a customer service advisor from Cranbrook, Exeter, first noticed a slightly dark veiny patch on her son's skin hours after he was born.
Within weeks, Sonny's birthmark had ballooned to a large, dark red lump, and at just six months old the blemish was the size of a snooker ball.
Explaining strangers' reactions to his mark, she said: "One person once shouted 'urgh, oh my God, what is wrong with your baby?' and another person came and asked if Sonny had cancer.
"I took Sonny to McDonald's once and there was a woman pointing him out to her children. It is hard to believe adults can be so cruel to a toddler.
"I tell Sonny that his birthmark means he is special, and that he has been kissed by an angel.
"Sonny is the happiest and most beautiful little boy, and it breaks my heart to think that this is something he might have to live with for the rest of his life.
"He is such a confident and smiley baby – I don't want that taken away from him just because some people can be shallow."
Doctors at Exeter hospital diagnosed Sonny with a capillary hemangioma just weeks after he was born.
Sonny was prescribed beta blockers and other medication to prevent the growth of the birthmark, but his mother says they did not have much effect.
Capillary hemangiomas are a tumours consisting of an abnormal overgrowth of tiny blood vessels.
They may not be present at birth, but appear within the first six months of life.
They usually begin to decrease in size between 12 and 15 months of age. Most regress nearly completely by five or six years of age.
Sonny is about to undergo an operation to de-bulk his birthmark, and Dennu hopes it will shrink as he gets older.
Denny said: "His face will never be symmetrical and perfect - he will always have scarring but to me he will always be the most beautiful thing I have ever laid eyes on.
"I want people to think before they speak. Sonny has feelings and we do as parents.
"It is something that will effect Sonny for the rest of his life."