© 2018 American Physical Society. The electronic structure of interstitial hydrogen in a compound semiconductor FeS2 (naturally n type) is inferred from a muon study. An implanted muon (Mu, a pseudohydrogen) forms electronically different defect centers discerned by the hyperfine parameter ωhf. A body of evidence indicates that one muon is situated at the center of an iron-cornered tetrahedron with nearly isotropic ωhf (Mup) and that the other exists as a diamagnetic state (Mud, ωhf≃0). Their response to thermal agitation indicates that the Mud center accompanies a shallow level (donor or acceptor) understood by the effective mass model, while the electronic structure of the Mup center is more isolated from the host than Mud, forming a deeper donor level. These observations suggest that interstitial hydrogen also serves as an electronically active impurity in FeS2. Based on earlier reports on the hydrogen diffusion in FeS2, the possibility of fast diffusion for Mup leading to the formation of a complex defect state (Mud∗, T≤100 K) or to a motional narrowing state (Mup∗, T≥150 K) is also discussed.