Apparently, Calvin believes that Santa Claus exists. Occasionally, Calvin asks himself if Santa might just be some kind of hoax, however, Hobbes tends to remind Calvin not to question Santa's presence, as doing so would hinder Calvin from getting the desired presents.
Hobbes also abuses Calvin's belief for his own good; he had almost brought Calvin to make a tuna sandwich for him.
Calvin seems to be the only character in the books that believes in Santa Claus. It is never clearly specified, but neither Hobbes nor Susie have ever mentioned anything of their opinion toward Santa, though Susie did comment on Calvin throwing a snowball at her once, claiming "Santa is going to skip your house for that", and Hobbes has gone with Calvin to try to see him.
Calvin often writes letters to Santa Claus. In these Calvin usually states why he should get presents or what Santa is doing wrong.