  We present a deep color-magnitude diagram in the VI passbands of the globular
cluster M54, a member of the Sagittarius dwarf galaxy. The data extend below
the cluster's main sequence turn-off, allowing us to estimate the cluster's
age. We find that M54 is 0.5--1.5 gigayears older than the Galactic globulars
M68 and M5. In absolute terms, the age is comparable to the published age
estimates of the other member clusters Arp 2 and Terzan 8, but is significantly
older than the member cluster Terzan 7. An age estimate of the Sagittarius
field population relative to M54 suggests that M54 is \gtrsim 3 Gyr older than
the field. We discuss briefly the star formation history of the Sagittarius
dwarf galaxy.

Astrophysics