The Center for Applied Research in the Apostolate at Georgetown University found between 1980 and 2012, the number of Catholic priests fell by more than 56,000.
It's prompted Rome to look abroad to fill its empty positions.
Abuse and corruption scandals and a decline in church attendance are thought to be contributing to the decrease.
But overall numbers appear to be steady, with the number of Catholic clergy in Africa and Asia more than doubling in the over the same time period.
Specifically, priests in Africa went up by 131%, while priests in Asia went up by 121% - an equivalent of more than 56,000 joining the clergy.
The report said: "... the Church in Europe will struggle in the future to provide access to Masses and sacraments in its many parishes given its rapidly declining population of priests. This will likely negatively impact levels of sacramental practice that have already been ebbing in recent years.
"In Africa, high fertility rates and expanding life expectancies will dramatically increase the number of Catholics from 198.6 million now to 460.4 million in 2040.
"In Asia, the Catholic population is expected to grow from 134.6 million now to 192.6 million in 2040... Unlike most other regions, the Church in Asia has experienced growing numbers of marriages in the Church. In 1980 there were about 381,700 marriages celebrated in Asian parishes. In 2012, 626,380 marriages were celebrated here."
