Phyllodes tumour was first described in 1838 by Johannes Muller. These tumours are uncommon and comprise < 0.5% of all breast neoplasms. Among the three histological subtypes-benign, borderline and malignant-the malignant variety is the most uncommon. Giant phyllodes tumours measure > 10 cm in their largest dimension. Overall prognosis for these lesions is poor, with high recurrence rates. Surgery with post-operative adjuvant chemoradiotherapy is the main treatment for malignant giant phyllodes tumours. We present a rare case of malignant giant phyllodes tumour of the left breast in a 23-year-old female patient with retrosternal extension and invasion of the pericardium.