There are two major theories on the origin of the Rosary. The traditional theory attributes the origin to St. Dominic in the 13th century. The Catholic Encyclopedia says that “the Feast of the Holy Rosary informs us that, when the Albigensian heresy was devastating the country of Toulouse, St. Dominic earnestly besought the help of Our Lady and was instructed by her…to preach the Rosary among the people as an antidote to heresy and sin.”
The second theory is that, beginning in the 12 century, the Rosary developed in imitation of the 150 biblical Psalms that priests and religious were required to pray over a certain period of time. Some monks even prayed all 150 Psalms every day. The 150 Psalms made up the “Psalter,” part of the Breviary, the Church’s official daily prayers. According to this theory, the Rosary developed as a substitute for the Breviary for those faithful who didn’t have the money to learn to read, but nevertheless desired a regular means by which they could express their devotion to Jesus and Mary through memorized prayers.
Adapted from Catholic Exchange “History of the Rosary”