Born at Prague in the year 1200; died probably in 1281. She was the daughter of Ottocar, King of Bohemia and Constance of Hungary, a relative of St. Elizabeth. At an early age she was sent to the monastery of Treinitz, where at the hands of the Cistercian religious she received the education that became her rank. She was betrothed to Frederick II, Emperor of Germany; but when the time arrived for the solemnization of the marriage, it was impossible to persuade her to abandon the resolution she had made of consecrating herself to the service of God in the sanctuary of the cloister. The Emperor Frederick was incensed at the unsuccessful issue of his matrimonial venture, but, on learning that St. Agnes had left him to become the spouse of Christ, he is said to have remarked: "If she had left me for a mortal man, I would have taken vengeance with the sword, but I cannot take offence because in preference to me she has chosen the King of Heaven." The servant of God entered the Order of St. Clare in the monastery of St. Saviour at Prague, which she herself had erected. She was elected abbess of the monastery, and became in this office a model of Christian virtue and religious observance for all. God favoured her with the gift of miracles, and she predicted the victory of her brother Wenceslaus over the Duke of Austria. The exact year of her death is not certain; 1281 is the most probable date.
(Taken from Catholic Encyclopedia)