The "Shape of You" hitmaker is considered a close friend of Blunt, 43, who is married to Sofia Wellesley. The couple's son was born last year.
"James is like a brother to me, actually I'm not quite sure if I can say this, but I'm going to he's just made me godfather to his son," Sheeran told The Sun.
"To be honest, I think he's only done it because he knows I've got a good idea of what sort of person he'd want him to be when he grows up an absolute lad. We're going to be brothers for life," he added.