Hidradenitis suppurativa (HS) is a chronic recurrent inflammatory condition presenting with painful, deep-seated abscesses and sinus tracts in multifocal locations. Rarely, longstanding inflammation in HS may lead to serious complications, such as cutaneous squamous cell carcinoma (SCC) (also termed Marjolin ulcer). Herein, we report a case of invasive cutaneous SCC arising from chronic ulcers of a HS patient. A 40-year old Korean male, a current smoker with 20 pack-year history, presented with a history of painful, recurrent, deep-seated abscesses and ulcers on the buttocks since his late teens, thus classified as Hurley stage III. A large purulent ulcer developed on the right buttock several months ago. Initial treatment was focused on controlling infection and facilitating wound healing. The lesion showed 50% reduction of size in 6 weeks, but also developed foul odor and showed fungating margins. Multiple skin biopsies were consistent with invasive SCC. Magnetic resonance imaging revealed a few enlarged lymph nodes on the right inguinal area, which was confirmed as metastasis on frozen biopsy. Slow Mohs micrographic surgery and radical right inguinal lymph node dissection was done. Incidence rates of SCC arising from HS have been reported up to 4.6%. To our knowledge, this is the first report of cutaneous SCC arising from HS in Korea. Our case emphasizes that the diagnosis of cutaneous SCC in HS should not be delayed, and early surgical intervention is crucial for better outcomes. (Ann Dermatol 33(1) 68∼72, 2021)