The spontaneous breaking of time-reversal symmetry is a hallmark of unconventional superconductivity, typically observed in type-II superconductors. Here, we report evidence of time-reversal symmetry breaking in the type-I superconductor ${\mathrm{YbSb}}_2$. Zero-field muon spin relaxation and rotation ($\mu \mathrm{SR}$) measurements reveal spontaneous internal magnetic fields emerging just below the superconducting transition, while transverse-field $\mu \mathrm{SR}$ confirms a fully gapped type-I superconducting state. Our first-principles calculations identify ${\mathrm{YbSb}}_2$ as a ${\mathbb{Z}}_2$ topological metal hosting a Dirac nodal line near the Fermi level. Symmetry analysis within the Ginzburg-Landau framework indicates an internally antisymmetric nonunitary triplet (INT) state as the most probable superconducting ground state. Calculations based on an effective low-energy model further suggest that this INT state may host gapless Majorana surface modes, pointing to the possibility of topological superconductivity in ${\mathrm{YbSb}}_2$. Our results highlight ${\mathrm{YbSb}}_2$ as a unique material platform where type-I superconductivity coexists with triplet pairing and nontrivial topology.

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• Superconductivity
• Topological materials
• Density functional theory
• First-principles calculations
• Muon spin relaxation & rotation