  Employing the unified first law of thermodynamics and the field equations of
the generalized Rastall theory, we get the generalized Misner-Sharp mass of
spacetimes for which $g_{tt}=-g^{rr}=-f(r)$. The obtained result differs from
those of the Einstein and Rastall theories. Moreover, using the first law of
thermodynamics, the obtained generalized Misner-Sharp mass and the field
equations, the entropy of the static spherically symmetric horizons is also
addressed in the framework of the generalized Rastall theory. In addition, by
generalizing the study to the flat FRW universe, the apparent horizon entropy
is also calculated. Considering the effects of applying the Newtonian limit to
the field equations on the coupling coefficients of the generalized Rastall
theory, our study indicates $i$) the obtained entropy-area relation is the same
as that of the Rastall theory, and $ii$) the Bekenstein entropy is recovered
when the generalized Rastall theory reduces to the Einstein theory. The
validity of the second law of thermodynamics is also investigated in the flat
FRW universe.
