It's reported Sisters from the Pontifical Institute of Foreign Missions were violently assaulted by a gang of up to 60 armed men.
They targeted the Catholic mission in Boldipukur on July 7 at 2am, attacking the presbytery, the convent and the hospital.
Bishop Sebastian Tudu said the attack was targeted and an attempt at intimidation.
He told Catholic charity Aid to the Church in Need: "The attack was massive and lasted about an hour and a half.
The attackers brutally beat the nuns. But we don't yet have the details. At the moment the nuns are in Dhaka for medical treatment.
"The door to the presbytery was broken down, and the pastor was threatened and robbed. The convent was seriously devastated. Only when the police arrived did the attackers leave the mission."
Reports suggest 12 people have been arrested.
It's thought to be the first time the nuns have been targeted but Bishop Tudu said it's left other Christians in fear.
Church reports suggest the incident was motivated by an attempt to steal the Sisters' land or other property deeds left with them for safekeeping by poor and illiterate locals.