  The Mn-based superconductor is rare owing to the strong magnetic
pair-breaking effect. Here we report on the discovery of pressure-induced
superconductivity in KMn6Bi5, which becomes the first ternary Mn-based
superconductor. At ambient pressure, the quasi-one-dimensional KMn6Bi5 is an
antiferromagnetic metal with TN = 75 K. By measuring resistivity and ac
magnetic susceptibility under hydrostatic pressures up to 14.2 GPa in a cubic
anvil cell apparatus, we find that its antiferromagnetic transition can be
suppressed completely at a critical pressure of Pc = 13 GPa, around which bulk
superconductivity emerges and displays a superconducting dome with the maximal
Tconset = 9.3 K achieved at about 14.2 GPa. The close proximity of
superconductivity to a magnetic instability in the temperature-pressure phase
diagram of KMn6Bi5 and an unusually large {\mu}0Hc2(0) = 18.9 T exceeding the
Pauli limit suggests an unconventional magnetism-mediated paring mechanism. In
contrast to the binary MnP, the flexibility of the crystal structure and
chemical compositions in the ternary AMn6Bi5 (A = alkali metal) can open a new
avenue for finding more Mn-based superconductors.
