The main two prayers of the Rosary are the Our Father and the Hail Mary. I assume no one will dispute the Our Father as not being biblical. The first stanza of the Hail Mary is:”Hail Mary, full of grace the Lord is with thee.” See Luke 1:28, “Hail, O favored one, the Lord is with you.” The phrase “O favored one” was translated “full of grace” in the Douay Rheims translation. Ok, so that first stanza is biblically based.
The second stanza is, “Bless art thou among women and blessed is the fruit of thy womb, Jesus. See Luke 11:27, “Blessed is the womb that bore you and the breasts that you sucked. Here wew see that second stanza is also biblically based.
The last stanza, “Holy Mary, Mother of God,pray for us sinners now and at the hour of our death, Amen.” The term Mother of God comes from the Council of Chalcedon. This last stanza reflects the belief of Catholics and the Orthodox on the effectiveness of requesting intercessory prayers of the saints. See also 11 Maccabees. Obviously, you and I will disagree on that point.
Oh; it's biblical alright.
But did not the disciples ask Jesus to teach them HOW to pray?
And not WHAT to pray?