  We study generalized Misner-Sharp energy in $f(R)$ gravity in a spherically
symmetric spacetime. We find that unlike the cases of Einstein gravity and
Gauss-Bonnet gravity, the existence of the generalized Misner-Sharp energy
depends on a constraint condition in the $f(R)$ gravity. When the constraint
condition is satisfied, one can define a generalized Misner-Sharp energy, but
it cannot always be written in an explicit quasi-local form. However, such a
form can be obtained in a FRW universe and for static spherically symmetric
solutions with constant scalar curvature. In the FRW universe, the generalized
Misner-Sharp energy is nothing but the total matter energy inside a sphere with
radius $r$, which acts as the boundary of a finite region under consideration.
The case of scalar-tensor gravity is also briefly discussed.
