It is "impossible" for God to "bless sin", the Congregation for the Doctrine of the Faith (CDF) said on Monday.
But the CDF did note the "positive elements" in same-sex relationships.
In October, Pope Francis said in a documentary that he thought same-sex couples should be allowed to have "civil unions".
In the Catholic Church, a blessing is given by a priest or other minister in the name of the Church.
