The electrodeposition of Ag at MoSe2 has been investigated, in situ, by electrochemical scanning tunneling microscopy (EC-STM). The process was investigated by observing MoSe2 at fixed electrode potentials which were referenced to the onset potential in the cyclic voltammograms of Ag electrodeposition at MoSe2. When an electrode potential more negative than the onset potential was applied to MoSe2, continuous Ag bulk deposition occurred. Consistently, the scanning tunneling microscopy (STM) image showed Ag deposits which exhibit a growth mode between island and layer growth. In the STM images taken near the onset potential, a small amount of Ag, presumably with sub-monolayer level coverage, was thought to be deposited. This initial deposition occurred so as to make the MoSe2 surface smooth in the STM images. STM imaging was difficult in the electrode potential region positive of the onset potential because the applied bias voltage became small. However, the STM images acquired during Ag deposition after applying such an electrode potential exhibited a characteristic morphological change, from which we infer that those processes were related to the MoSe2 reaction with Ag.