  Electronic and optical excitations in two-dimensional moir\'e systems are
uniquely sensitive to local atomic registries, leading to materials- and
twist-angle specific correlated electronic ground states with varied degree of
localization. However, there has been no direct experimental correlation
between the sub-nanometer structure and emergent excitonic transitions,
comprising tightly-bound pairs of photoexcited electrons and holes. Here, we
use cryogenic transmission electron microscopy and spectroscopy to
simultaneously image the structural reconstruction and associated localization
of the lowest-energy intralayer exciton in a rotationally aligned
heterostructure of WS2 and WSe2 monolayers. In conjunction with optical
spectroscopy and ab initio calculations, we determine that the exciton
center-of-mass wavefunction is strongly modulated in space, confined to a
radius of ~ 2 nm around the highest-energy stacking site in the moir\'e
unit-cell, forming a triangular lattice. Our results provide direct evidence
that atomic reconstructions lead to the strongly confining moir\'e potentials
and that engineering strain at the nanoscale will enable new types of excitonic
lattices.
