A gene encoding a starch-hydrolyzing enzyme was isolated from a marine metagenomic library and overexpressed in Escherichia coli. The enzyme, designated AmyP, shows very low similarity to full-length sequences of known α-amylases, although a catalytic domain correlated with the α-amylase superfamily was identified. Based on the range of substrate hydrolysis and the product profile, the protein was clearly defined as a saccharifying-type α-amylase. Sequence comparison indicated that AmyP was related to four putative glycosidases previously identified only in bacterial genome sequences. They were all from marine bacteria and formed a new subfamily of glycoside hydrolase GH13. Moreover, this subfamily was closely related to the probable genuine bacterial α-amylases (GH13_19). The results suggested that the subfamily may be an independent clade of ancestral marine bacterial α-amylases.