Let D be a generalized dihedral group and Aut Col (D) its Coleman automorphism group. Denote by Out Col (D) the quotient group of Aut Col (D) by Inn(D), where Inn(D) is the inner automorphism group of D. It is proved that either Out Col (D) = 1 or Out Col (D) is an elementary abelian 2-group whose order is completely determined by the cardinality of π(D). Furthermore, a necessary and sufficient condition for Out Col (D) = 1 is obtained. In addition, whenever Out Col (D) = 1, it is proved that Aut Col (D) is a split extension of Inn(D) by an elementary abelian 2-group for which an explicit description is given.

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