In its traditional form, tofu is vegan. Any product that is not derived from animals is vegan-friendly. In other words, any item made from fully plant-based ingredients is vegan. Tofu is produced with soy milk and it is vegan. Your doubt is understandable as tofu looks like a dairy item. Soy milk is made with soybeans. It is a non-dairy product, with no animal-based ingredient in it. Therefore, traditional tofu is vegan.
 
Speaking of ingredients, there are tofu products with eggs and other animal-related things added to these. So, as a customer, you have to look at the ingredients to ensure that what you buy is vegan. Some think that tofu is not vegan because of the non-vegan ingredients in some tofu products. I have heard that tofu is an excellent source of protein for vegetarians and vegans alike.