An elongated support post for a mailbox has a mailbox support arm secured to an upper portion of the support post and at least one anchor fin secured to a lower portion of the support post and projectingly outwardly therefrom. In one embodiment of the invention a pair of support fins is secured to generally opposed portions of the support post and project radially outwardly therefrom. The support post may have openings into which portions of the support fins extend for mechanical interengagement therebetween. The support fins are preferably taperedly downwardly so as to facilitate insertion of the post into the earth.