We report on a 9-month-old female infant with multiple tense bullae and erosions covering the entire body, including the face, scalp, and trunk. The histopathological examination revealed sub-epidermal bullae with a dense dermal cellular infiltrate. The infiltrate was identified as a collection of mast cells using toluidine blue and Giemsa stains. The direct immunofluorscence was negative. A diagnosis of cutaneous diffuse mastocytosis with generalized bullae was made based on these clinical and histopathological findings. In cases with diffuse cutaneous mastocytosis with generalized bullae, systemic involvement is more frequent and more severe compared to other types of cutaneous mastocytosis. Some lethal outcomes have been reported. This is the first reported case of diffuse cutaneous mastocytosis in the Korean literature.