We have studied the electronic properties of ScS, a transition-metal monochalcogenide with a rocksalt crystal structure, using magnetization, specific heat, transport, and muon spin rotation/relaxation ($\mu \mathrm{SR}$) measurements. All measurements confirm the bulk superconductivity in ScS with a transition temperature of $T_C=5.1\left(5\right)$ K. Specific heat together with transverse-field $\mu \mathrm{SR}$ measurements indicate a full gap, while our zero-field $\mu \mathrm{SR}$ study reveals the presence of spontaneous static or quasistatic magnetic fields emerging when entering the superconducting state. We discuss various possible microscopic origins of the observed time-reversal symmetry breaking. As none of them can be readily reconciled with a conventional pairing mechanism, this introduces ScS as a candidate material for unconventional superconductivity.

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• Unconventional superconductors
• Muon spin relaxation & rotation
• T-symmetry