  We discover chiral phonons at the lowest-energy bands in moir\'e
superlattices. The moir\'e chiral phonons we uncover are the collective
excitations of the stacking domains. Their origin is uniquely attributed to the
stacking configurations whose interlayer binding energy breaks the $C_{2z}$
symmetry on the moir\'e length scale. Within elastic theory, we use a general
symmetry analysis to provide a complete classification of van der Waals
heterostructures in respect to hosting moir\'e chiral phonons and show the
calculation for twisted MoS$_2$ as an example. We present a low-energy
effective model to qualitatively understand the moir\'e chiral phonons and show
that it captures the essential physics remarkably well. Our result potentially
opens up new possibilities in phononic twistronics as the moir\'e chiral
phonons have high tunability, moir\'e scale wavelengths, excitation energies in
only a few meV, and can possibly be mechanically excited.
