Salvinia pseudoformosa has been established as a Miocene species based mainly on specimens collected from the Sasakizawa River, Numata, Hokkaido, Japan. However, both Eocene and Miocene sediments are distributed within this area. In this study, we conducted geological surveys along the Sasakizawa River to identify the stratigraphic horizon bearing S. pseudoformosa and found that S. pseudoformosa occurs from the middle Eocene Itarakaomappu Formation of the Uryu Group. We also reexamined syntypes of this species and concluded that only those from the Sasakizawa River can be identified as S. pseudoformosa with certainty. Therefore, we designated a syntype from the Sasakizawa River as the lectotype. Although we could not specify the exact localities of the lectotype and remaining syntypes from the Sasakizawa River, lithological characters of the slabs containing these specimens indicate their occurrence from the Uryu Group. Salvinia pseudoformosa has been recorded from the middle Miocene in Japan and Korea, but the venation patterns of these specimens differ from those of S. pseudoformosa.