A novel structure of water aggregate is revealed by means ofscanning tunneling microscopy (STM). We find that water molecules are self-assembled into one-dimensional chains at 78 K on Cu(110). Reflecting the two-fold symmetry of the surface, the chain grows exclusively along the direction perpendicular to the Cu rows. The chain exhibits a zigzag structure in the STM images with a period of 7.2 Å, and grows to a length of ∼1,000 Å. This is incontrast to the two-dimensional water overlayer on close-packed metallic surfaces of three-fold symmetry. We propose that water hexamers mainly constitute the zigzag 1D chain. Two-dimensional overlayer appears only at high coverage.