A post anchor for securing a support post for a structure comprises a base with four upstanding sidewalls forming a cavity into which a post is inserted, and means to secure the post inside the cavity. Interior ribs and exterior augmented corners resist deformation under pressure exerted by the post. Interior channels allow water to drain from inside the cavity toward a drain hole. The anchor post is slightly elevated relative to the ground, allowing air and water to circulate around the post and post anchor. The post anchor is made of a thermoplastic polymer which is given a textured surface, and may include an ultraviolet protection additive.