A post sleeve includes a reinforced concrete body preformed around a liner that defines a cavity extending longitudinally within the body, sized to receive a post. Standoff ribs run lengthwise within the cavity and extend inward from inner walls of the cavity. A post in the cavity is supported laterally by the standoff ribs. Drain channels between the ribs permit water to flow past the post and exit the cavity via a lower aperture. A drain tube is coupled to the lower aperture, and extends downward where it is covered with gravel at the bottom of a post hole. Concrete is poured around the post sleeve in the hole. The cavity is adaptable to receive posts of varying sizes, and at various depths. A collar closes a space between the post and the top of the cavity, permitting air circulation within the cavity while shedding water and substantially preventing insects from entering the cavity.