  Weyl semimetals are well-known for hosting topologically protected linear
band crossings, serving as the analog of the relativistic Weyl Fermions in the
condensed matter context. Such analogy persists deeply, allowing the existence
of the chiral anomaly under parallel electric and magnetic field in Weyl
semimetals. Different from such picture, here we show that, a unique mechanism
of the chiral anomaly exists in Weyl semimetals by injecting a spin current
with parallel spin polarization and flow direction. The existence of such a
chiral anomaly is protected by the topological feature that each Weyl cone can
also be a source or drain of the spin field in the momentum space. It leads to
measurable experimental signals, such as an electric charge current parallel
with an applied magnetic field in the absence of the electric field, and a
sharp peak at certain resonant frequency in the injection current in achiral
Weyl semimetals through the circular photogalvanic effect. Our work shows that
the topological implication of Weyl semimetals goes beyond the link with
relativistic Weyl Fermions, and offers a promising scenario to examine the
interplay between topology and spin.
