Observations by the Sounding of the Atmosphere using Broadband Emission Radiometry instrument on the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics satellite from January 2002 to June 2014 are used to study the vertical structure of OH dayglow. The results indicate for the first time that there is a double-layer structure in the distributions of 12?year averaged OH airglow emission, [O3], and [H] during the daytime. The upper layer of OH dayglow is located in the mesopause region (~88?km) at a similar altitude to that of the OH nightglow. The lower layer is situated in the range of 70–85?km. Both the peak emission and height of the lower layer increase with local time. The distance between the two layers decreases with local time. At the equator, the lower layer forms at ~09:00 LT and lasts for about 8?h; during this time the interlayer distance decreases from 13?km to 5?km. The double-layer structure is more obvious and longer-lived during the equinoxes and at lower latitudes. The double-layer structure of OH dayglow emission is a long-term stable structure and is mainly caused by photochemical processes involving [O3]. It is also modulated by background atmospheric temperature and [H].