Determination of the adsorption structure of water molecules on metal surfaces is an imperative challenge to understanding the mechanisms of the wetting process and water-related heterogeneous catalysis. We identify water monolayers formed on Ni(111) via low-temperature atomic force microscopy, which enables the visualization of individual water molecules in monolayers with higher spatial resolution than scanning tunneling microscopy. On the terraces of Ni(111) at 150 K, water forms monolayers comprising fused pentagonal, hexagonal, and heptagonal rings. Water adsorbates on step sites assemble in a different manner, forming a hydrogen-bonding network with fused pentagonal and octagonal rings aligned in the step direction. Because similar water networks with pentagonal rings have been proposed in monolayers or their defect sites on other metal surfaces, our structural characterization of H2O/Ni(111) provides an insight into water adsorption structures on metals.