Brian Oge O'Rourke (Irish: Brian Óg na Samhthach Ó Ruairc; c. 1568 – 28 January 1604), was the penultimate king of West Breifne, from 1591 until his overthrow in April 1603, at the end of the Nine Years' War.
In 1599, O'Rourke's forces fought alongside those of Hugh Roe O'Donnell at the Battle of Curlew Pass, during the Nine Years' War.
His forces, along with those of Hugh O'Neill, 2nd Earl of Tyrone, were still sufficiently menacing to Queen Elizabeth I that she was persuaded to agree to a peace in Ireland - the Treaty of Mellifont.
O'Rourke was the last Irish king to be defeated in the war, roughly a month after the others had surrendered.
He never surrendered, but was ousted by his brother Teigue, who had defected to the English during the war and with their support invaded his kingdom in March 1603.