We investigated the atomic and electronic structures of interstitial hydrogen impurity in rutile TiO2 and anatase TiO2 through first-principles pseudopotential calculations. The interstitial H atom is attached to the nonbonding p-orbital of an oxygen atom, forming a hydroxyl ion. In rutile TiO2, it is located between two oxygens on an ab-plane. In anatase TiO2, the O-H direction is located along a cubic a-direction. The H is found to be electrically positively charged, and the H impurity level is located above the conduction band minimum, indicating that the H-int is a shallow donor-like impurity. Its contamination should lead to n-type conductivity.