  Many-body interactions between carriers lie at the heart of correlated
physics. The ability to tune such interactions would open the possibility to
access and control complex electronic phase diagrams on demand. Recently,
moir\'e superlattices formed by two-dimensional materials have emerged as a
promising platform for quantum engineering such phenomena. The power of the
moir\'e system lies in the high tunability of its physical parameters by
tweaking layer twist angle, electrical field, moir\'e carrier filling, and
interlayer coupling. Here, we report that optical excitation can drastically
tune the spin-spin interactions between moir\'e trapped carriers, resulting in
ferromagnetic order in WS2/WSe2 moir\'e superlattices over a small range of
doping at elevated temperatures. Near the filling factor v = -1/3 (i.e., one
hole per three moir\'e unit cells), as the excitation power at the exciton
resonance increases, a well-developed hysteresis loop emerges in the reflective
magnetic circular dichroism (RMCD) signal as a function of magnetic field, a
hallmark of ferromagnetism. The hysteresis loop persists down to charge
neutrality, and its shape evolves as the moir\'e superlattice is gradually
filled, indicating changes of magnetic ground state properties. The observed
phenomenon points to a mechanism in which itinerant photo-excited excitons
mediate exchange coupling between moir\'e trapped holes. This exciton-mediated
interaction can be of longer range than direct coupling between moir\'e trapped
holes, and thus magnetic order can arise even in the dilute hole regime under
optical excitation. This discovery adds a new and dynamic tuning knob to the
rich many-body Hamiltonian of moir\'e quantum matter.
