Isolated water molecules such as monomers and dimers on Pt(111) surfaces with low degree of coverage were directly observed by scanning tunneling microscopy (STM) at low temperatures. A water monomer is shown as a single protrusion in the STM image. While, a dimer appears as a 6-fold symmetric “flower-like” protrusion, which can be explained by the model that one of the water molecules in the dimer is rotating around the other. The adsorption sites of a dimer were also determined as both molecules on atop sites. Indeed, we have succeeded in forming and breaking dimers by exciting molecular vibrations with inelastically tunneled electrons from an STM tip. It turns out that a dimer is more stable than a monomer when the sample bias voltage is shifted. We also report on vibrationally induced lateral hopping motion of both monomer and dimer molecules by injecting tunneling electron.