Single-atom catalysts (SACs) play a vital role in the hydrogen evolution reaction (HER) owing to the highly desirable atom efficiency and the minimal amount of precious metals. Herein, we use TiO2 nanosheets to anchor stable atomically dispersed iridium (Ir) to be used as a catalyst (Ir@TiO2) for the HER. The atomic dispersion of Ir on the TiO2 substrate is confirmed by aberration-corrected scanning transmission electron microscopy and it is anchored by numerous surface functional groups on abundantly exposed basal planes in TiO2. In acidic media, the Ir@TiO2 catalyst (1.35 wt% Ir) shows a low overpotential (41 mV at 10 mA cm-2), a small Tafel slope of 42 mV dec-1, and a decent durability for 1000 cycles of the HER with the polarization curve having only a 1 mV shift, which are comparable with those of a commercial Pt/C catalyst with 20 wt% Pt. This work paves a way to design Ir atomically anchored catalysts with low cost and high activity.