  The discovery of superconductivity in twisted bilayer and twisted trilayer
graphene has generated tremendous interest. The key feature of these systems is
an interplay between interlayer coupling and a moir\'e superlattice that gives
rise to low-energy flat bands with strong correlations. Flat bands can also be
induced by moir\'e patterns in lattice-mismatched and or twisted
heterostructures of other two-dimensional materials such as transition metal
dichalcogenides (TMDs). Although a wide range of correlated phenomenon have
indeed been observed in the moir\'e TMDs, robust demonstration of
superconductivity has remained absent. Here we report superconductivity in 5
degree twisted bilayer WSe$_2$ (tWSe$_2$) with a maximum critical temperature
of 426 mK. The superconducting state appears in a limited region of
displacement field and density that is adjacent to a metallic state with Fermi
surface reconstruction believed to arise from antiferromagnetic order. A sharp
boundary is observed between the superconducting and magnetic phases at low
temperature, reminiscent of spin-fluctuation mediated superconductivity. Our
results establish that moir\'e flat-band superconductivity extends beyond
graphene structures. Material properties that are absent in graphene but
intrinsic among the TMDs such as a native band gap, large spin-orbit coupling,
spin-valley locking, and magnetism offer the possibility to access a broader
superconducting parameter space than graphene-only structures.
