Hydrogen can be inserted into Si(100)-2 × 1-H during surface preparation or during the hydrogen desorption lithography used to create atomic-scale devices. Here, a hydrogen atom inserted into a hydrogen monolayer on the Si(100)-2 × 1 surface has been studied using density functional theory. Hydrogen-induced defects were considered in their neutral, negative, and positive charge states. It was found that hydrogen forms a dihydride unit on the surface in the most stable neutral and negative charge states. Hydrogen located in the groove between dimer rows is also one of the most stable negative charge states. In the positive charge state, hydrogen forms a three-center bond inside a Si dimer, Si-H-Si, similar to the bulk case. A comparison of simulated scanning tunneling microscopy (STM) images with the experimental data available in the literature showed that neutral and negatively charged hydrogen-induced defects were already observed in experiments. The results reveal that the H atom inserted into a hydrogen monolayer on the Si(100)-2 × 1 surface can lead to the formation of a positively or negatively charged defect. It is shown that H atoms in the considered configurations can play a role in various surface reactions.