Sisters Mary Kreuper and Lana Chang, who worked at St James’ Catholic School in the city of Torrance, California, stole the money from school funds and gambled with it in casinos.
The nuns, said to be best friends, took money from an account used for tuition fees and donations, and were caught during a routine audit.
They had previously avoided detection by depositing cheques made out to the school into their own separate bank account.
The sisters, who have just retired, have expressed remorse for their actions, and although the police have been informed, the school is not bringing criminal charges against them.
Sister Mary was the principal of the school for 29 years, while Sister Lana was a teacher there for about 20 years.
It is thought they spent the stolen money over a period of at least 10 years on gambling and travel.
In a statement, the Sisters of St Joseph of Carondelet, the pair’s order, said: “The Sisters have confirmed the misappropriation of funds and have co-operated in the investigation.
“Our community is concerned and saddened by this situation and regret any injury to our long relationship with the families of the school.”
In a letter to parishioners, the monsignor of the church, Michael Meyers, wrote: “Sister Mary Margaret and Sister Lana have expressed to me and asked that I convey to you, the deep remorse they each feel for their actions and ask for your forgiveness and prayers.”
Parents of pupils at the school were reportedly told about the misappropriation of funds earlier this month.