The decree was written by the Vatican's Congregation for the Doctrine of the Faith and published in the Vatican newspaper L'Osservatore Romano, giving it immediate effect.
A Vatican spokesman said the decree made the Church's existing ban on women priests more explicit by clarifying that excommunication would follow all such ordinations.
Excommunication forbids those affected from receiving the sacraments or sharing in acts of public worship.
Rev. Tom Reese, a senior fellow at the Woodstock Theological Center at Georgetown University, said he thought the decree was meant to send a warning to the growing number of Catholics who favour admitting women to the priesthood.
"I think the reason they're doing this is that they've realised there is more and more support among Catholics for ordaining women, and they want to make clear that this is a no-no," Reese said.
The Church says it cannot change the rules banning women from the priesthood because Christ chose only men as his apostles. Church law states that only a baptised male can be made a priest. Proponents of women's ordination say Christ was only acting according to the social norms of his time.
They cite the letters of Saint Paul, some of the earliest texts of Christianity, to show that women played important roles in the early church.
Attempts to ordain women priests are highly unusual. But the archbishop of St. Louis earlier this year declared three women excommunicated after an ordination ceremony in his diocese.
Excommunication is usually "ferendae sententiae", imposed as punishment.
But some offences, including heresy, schism, and laying violent hands on the Pope, are considered so disruptive of ecclesiastical life that they trigger automatic excommunication, or "latae sententiae".
The decree says that women priests and the bishops who ordain them would be excommunicated "latae sententiae".
This was the same excommunication invoked against a renegade African archbishop who also broke Vatican rules when he ordained four married men bishops in 2006.
The archbishop, Emmanuel Milingo, made world headlines in 2001 for getting married himself in Reverend Sun Myung Moon's Unification Church at a mass wedding in a New York hotel. His union was never recognised by the Catholic Church.