In athletes, acute bacterial infection is an unusual cause of pain in the sacroiliac joint. Although an entry site for infection is not always evident, the present case of a 15-year-old rugby player suggests the association between right sacroiliac joint infection and skin lesion of atopic dermatitis (AD) infected with group A streptococcus. Magnetic resonance imaging revealed inflammation around the sacroiliac joint with abscess formation. The infection resolved after a course of antibiotics. Because atopic skin lesion is a potential portal of bacteria, treatment for AD is essential for the prevention of pyogenic arthritis in athletes.