  The newly discovered valley degree of freedom in atomically thin
two-dimensional transition metal dichalcogenides (TMD) offers a promising
platform to explore rich nonlinear physics, such as spinor Bose-Einstein
condensate (BEC) and novel valleytronics applications. However, the critical
nonlinear effect, such as valley polariton bosonic stimulation, has long
remained an unresolved challenge due to the generation of limited polariton
ground state densities necessary to induce the stimulated scattering of
polaritons in specific valleys. Here, we report the self-induced valley bosonic
stimulation of exciton-polaritons via spin-valley locking in a WS2 monolayer
microcavity. This is achieved by the resonant injection of valley polaritons at
specific energy and wavevector, which allows spin-polarized polaritons to
efficiently populate their ground state and induce a valley-dependent bosonic
stimulation. As a result, we observe the nonlinear self-amplification of
polariton emission from the valley-dependent ground state. Our finding paves
the way for the investigation of spin ordering and phase transitions in TMD
polariton BEC, offering a promising route for the realization of polariton spin
lattices in moir\'e polariton systems and spin-lasers.
