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 YbSb${}_2$. Zero-field $\mu$SR measurements reveal spontaneous internal magnetic fields emerging just below the superconducting transition, while transverse-field $\mu$SR confirms a fully gapped type-I superconducting state. Our first-principles calculations identify 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 YbSb${}_2$. Our results highlight YbSb${}_2$ as a unique material platform where type-I superconductivity coexists with triplet-pairing and nontrivial topology.