  The electron's kinetic energy plays a pivotal role in magnetism. While
virtual electron hopping promotes antiferromagnetism in an insulator, the real
process usually favors ferromagnetism. But in kinetically frustrated systems,
such as hole doped triangular lattice Mott insulators, real hopping has been
shown to favor antiferromagnetism. Kinetic frustration has also been predicted
to induce a new quasiparticle -- a bound state of the doped hole and a spin
flip called a spin polaron -- at intermediate magnetic fields, which could form
an unusual metallic state. However, the direct experimental observation of spin
polarons has remained elusive. Here we report the observation of spin polarons
in triangular lattice MoTe2/WSe2 moir\'e bilayers by the reflective magnetic
circular dichroism measurements. We identify a spin polaron phase at lattice
filling factor between 0.8-1 and magnetic field between 2-4 T; it is separated
from the fully spin polarized phase by a metamagnetic transition. We determine
that the spin polaron is a spin-3/2 particle and its binding energy is
commensurate to the kinetic hopping energy. Our results open the door for
exploring spin polaron pseudogap metals, spin polaron pairing and other new
phenomena in triangular lattice moir\'e materials.
