"Newly published statistics showed that the number of men and women belonging to religious orders fell by 10% to just under a million between 2005 and 2006.
During the pontificate of the late Pope John Paul II, the number of Catholic nuns worldwide declined by a quarter.
The downward trend accelerated despite a steady increase in the membership of the Catholic Church to more than 1.1bn.
However, correspondents say even this failed to keep pace with the overall increase in world population.
On the back page of its official newspaper, L'Osservatore Romano, the Vatican published on Monday new statistics revealing that between 2005 and 2006 the number "members of the consecrated life" fell by just under 10%.
The number of members, predominantly women, some engaged only in constant prayer, others working as teachers, health workers and missionaries, fell 94,790 to 945,210.
Of the total, 753,400 members were women, while 191,810 were men, including 136,171 priests and 532 permanent deacons.
The figures were published next to a report of Pope Benedict XVI's meeting with nuns, monks and priests from many countries gathered in St Peter's Basilica in Rome last weekend.
The BBC's David Willey in the Italian capital says the accelerating downward trend must have caused concern to the Pope.
The Roman Catholic Church has an aging and diminishing number of parish and diocesan clergy and this latest fall is quite dramatic, our correspondent says.
Jesús se supone le dijo a sus apostoles que si deseaban seguirlo dejarán a sus padres, pero nunca dijo nada sobre dejar a sus mujeres... A ver si con esta sangría por fin el patriarcado vaticano borrar esa tardía invención del celibato obligatorio, que ni los rabinos ortodoxos realizan, y si algo fue Jesús (si existió, claro) es un rabino...