The Collaborative International Dictionary of English v.0.48:

Southmost \South"most`\, a.
   Farthest toward the south; southernmost. [R.] --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

southmost
    adj 1: situated farthest south; "Key West is the southernmost
           city in the continental United States" [syn:
           southernmost, southmost]