Infantile seborrheic dermatitis often occurs during the first three months of life and most frequently presents as erythema and greasy scales located especially on the scalp (commonly called “cradle cap”). Usually, it is a mild, self-limiting condition. The severe, erythrodermic clinical appearance is rare and often demands differential diagnosis with other more serious skin conditions. We report a case of severe seborrheic dermatitis in an infant. A 5-weeks male infant presented with erythrodermic scaling lesions and exfoliation of the outermost layer of the epidermis, which had appeared after birth. Greasy scales were observed on the scalp, eyelids, and face accompanied by inflammation of the eyelids. Moreover, erythematous, well-demarcated lesions were noticed in the neck folds, behind the ears, in the axillary region, and diaper area. Considering the severe clinical appearance, additional tests such as skin biopsy and genetic analysis were performed to exclude other possible causes such as atopic dermatitis, Langerhans histiocytosis, congenital ichthyosis, and psoriasis. Based on clinical presentation and additional test results, infantile seborrheic dermatitis seemed to be the most probable diagnosis. The treatment including 1% tannic acid, 0.5% erythromycin eye cream, clotrimazole cream, hydrocortisone cream, and emollients was started in the hospital with a good response. After a month of therapy, the patient was re-admitted for the follow-up, with further improvement of the skin condition. It is essential to remember that the dermatoses that we should take into consideration during the differential diagnosis of severe infantile seborrheic dermatitis are atopic dermatitis, Langerhans histiocytosis, congenital ichthyosis, and psoriasis.