Idiopathic granulomatous mastitis (IGM) is a very rare clinical-histological entity that affects young women of childbearing age and represents a real diagnostic and therapeutic challenge for clinicians. Systemic manifestations during IGM are extremely rare. The most reported manifestations are: erythema nodosum, inflammatory arthralgia, fever, peripheral lymphadenopathies, and scleritis/episcleritis. Arthritis as extra-mammary manifestations of IGM remains exceptional, and revealing forms are unusual. We report an original case of IGM occurring in a pregnant woman, whose inaugural symptom was an acute and isolated monoarthritis of the left ankle.