  The creation of moir\'e patterns in crystalline solids is a powerful approach
to manipulate their electronic properties, which are fundamentally influenced
by periodic potential landscapes. In 2D materials, a moir\'e pattern with a
superlattice potential can form by vertically stacking two layered materials
with a twist and/or finite lattice constant difference. This unique approach
has led to emergent electronic phenomena, including the fractal quantum Hall
effect, tunable Mott insulators, and unconventional superconductivity.
Furthermore, theory predicts intriguing effects on optical excitations by a
moir\'e potential in 2D valley semiconductors, but these signatures have yet to
be experimentally detected. Here, we report experimental evidence of interlayer
valley excitons trapped in a moir\'e potential in MoSe$_2$/WSe$_2$
heterobilayers. At low temperatures, we observe photoluminescence near the free
interlayer exciton energy but with over 100 times narrower linewidths. The
emitter g-factors are homogeneous across the same sample and only take two
values, -15.9 and 6.7, in samples with twisting angles near 60{\deg} and 0\deg,
respectively. The g-factors match those of the free interlayer exciton, which
is determined by one of two possible valley pairing configurations. At a twist
angle near 20\deg, the emitters become two orders of magnitude dimmer, but
remarkably, they possess the same g-factor as the heterobilayer near 60\deg.
This is consistent with the Umklapp recombination of interlayer excitons near
the commensurate 21.8{\deg} twist angle. The emitters exhibit strong circular
polarization, which implies the preservation of three-fold rotation symmetry by
the trapping potential. Together with the power and excitation energy
dependence, all evidence points to their origin as interlayer excitons trapped
in a smooth moir\'e potential with inherited valley-contrasting physics.
