Armina is the most species-rich genus of the Arminidae family with over 50 nominal species. Material of the genus Armina from the western Atlantic and the eastern Pacific was revised. Six species have been documented from the western Atlantic; however, we have determined that only four of them are valid: Armina muelleri, A. wattla, A. juliana and A. elongata . Also, only three out of seven species previously registered in the eastern Pacific were recognized in the present study: A. californica, A. cordellensis and Armina sp., an unnamed species. The phylogenetic analysis of 13 taxa and 17 characters was performed using the program PAUP (Phylogenetic Analysis Using Parsimony). The Branch-and-Bound algorithm generated a 29-step tree with the following relations: ( Histiomena convolvula , ( Dermatobranchus sp., ((((A. californica , ( A. maculata, A. muelleri )), (( A. loveni, A. neapolitana ), A. wattla )), ( A. cordellensis , ( Armina sp., A. juliana ))), ( A. tigrina, A. elongata )))). The monophyletic evidence for Armina is discussed and compared to possible speciation processes similar to those found in other Opisthobranchia groups.