A new type of muonium defect center has been observed in undoped CdS below 20 K. The hyperfine interaction amounts only to approximately ${10}^{-4\mathrm{}}$ of the vacuum value, and is shown to have axial symmetry along the Cd-S bond direction. Results suggest that the muon is close to the sulfur antibonding site and the paramagnetic electron density is distributed over a large volume. In contrast to the behavior in other semiconductors, muonium forms a shallow center in CdS. By implication, analog isolated hydrogen impurity atoms act as electrically active shallow-level defect centers in CdS.