Aprataxin is a DNA deadenylase that resolves DNA 5'-AMP termini and reverses abortive DNA ligation. The crystal structures of Schizosaccharomyces pombe aprataxin Hnt3 in its apo form and in complex to dsDNA and dsDNA-AMP reveal how Hnt3 recognizes and processes 5'-adenylated DNA in a structure-specific manner. The bound DNA adopts a 5'-flap conformation that facilitates 5'-AMP access to the active site, where AMP cleavage occurs by a canonical catalytic mechanism.