We show that in a generic scalar-tensor theory of gravity, the "referenced"
quasilocal mass of a spatially bounded region in a classical solution is i
nvariant under conformal transformations of the spacetime metric. We first
extend the Brown-York quasilocal formalism to such theories to obtain the '
'unreferenced'' quasilocal mass and prove it to be conformally invariant. H
owever, this quantity is typically divergent. It is, therefore, essential t
o subtract from it a properly defined reference term to obtain a finite and
physically meaningful quantity, namely, the referenced quasilocal mass. Th
e appropriate reference term in this case is defined by generalizing the Ha
wking-Horowitz prescription, which was originally proposed for general rela
tivity. For such a choice of reference term, the referenced quasilocal mass
for a general spacetime solution is obtained. This expression is shown to
be a conformal invariant provided the conformal factor is a monotonic funct
ion of the scalar field. We apply this expression to the case of static sph
erically symmetric solutions with arbitrary asymptotics to obtain the refer
enced quasilocal mass of such solutions. Finally, we demonstrate the confor
mal invariance of our quasilocal mass formula by applying it to specific ca
ses of four-dimensional charged black hole spacetimes, of both the asymptot
ically flat and non-flat kinds, in conformally related theories. [S0556-282
1(99)01402-1].