Purpose. This study aims to investigate the influence of the tidal-driven Okhotsk Sea water inflow through the La Perouse Strait on the hydrological conditions along the western coast of the Krillon Peninsula (Southwestern Sakhalin) during the warm season.

Methods and Results. The material for investigation is based on the instrumental measurement data on sea current velocities, sea level fluctuations, and seawater temperature and salinity near the Gorbusha River mouth collected in May ‒ October 2024. The results of oceanographic surveys performed in the coastal zone (up to the 20 m isobath) along transects perpendicular to the coast were also used. Additionally, the data on current velocities, water temperature, and salinity collected at the autonomous buoy station “Astarta”, located in the central part of the La Perouse Strait (May ‒ June 1999), as well as the meteorological observation data from the Nevelsk weather station (May ‒ October 2024) were utilized. Against the backdrop of diurnal fluctuations, a semi-monthly cyclicity of the Okhotsk Sea water intrusion was identified: a 10–11-day phase of intensified inflow to the Southwest Sakhalin shelf during periods of tropical tides, followed by a 3–4-day period of marked weakening during equatorial tides. Standard methods of statistical and harmonic analysis were applied to the observation data.

Conclusions. The area off the western coast of the Krillon Peninsula is significantly influenced by the Okhotsk Sea water entering through the La Perouse Strait, driven by strong diurnal tidal currents. The diurnal temperature fluctuations can reach 10 °C, and salinity variations can reach 2 psu. An unexpected finding of this study was the identification of not only lower salinity but also higher temperature in the Okhotsk Sea water compared to those of the West Sakhalin Current, which flows southward along the southwestern coast of Sakhalin.