We investigate the structural and electronic properties of hydrogen impurity in HgO through first-principles pseudo-potential total-energy calculations. The most stable position of the interstitial hydrogen is identified to be at the location between two O atoms, each of which is in two adjacent chains, forming a H-O bonding. Our electronic-structure calculations indicate that the interstitial hydrogen is a shallow-donor-type impurity. We also examined the formation enthalpy of the interstitial hydrogen. This indicates that HgO can be easily contaminated by H.