What is the ninth commandment of God? The ninth commandment of God is: Thou shalt not covet thy neighbor's wife.
Thou shalt not covet thy neighbor's wife. (Deuteronomy 5:21)
What are we commanded by the ninth commandment? By the ninth commandment we are commanded to be pure in thought and desire.
Blessed are the clean of heart, for they shall see God. (Matthew 5:8)
Deeper reading -> CCC 2514-2516