Lake St. Moritz, Alpine reservoir in the Upper Engadine, Canton of the Grisons, Switzerland.

Lake St. Moritz (St. Moritzersee) is the northeasternmost and second-smallest of the four lakes of the Upper Engadine, covering 0.78 km² at 1,768 m above sea level. It lies between Piz Rosatsch to the south and Piz Nair to the north, with St. Moritz town on the raised north bank and the mineral springs of St. Moritz-Bad at its western end. The Inn flows through the lake, and since 1932 two 14-metre-wide weirs at the outflow keep the water level constant, with a fish ladder allowing fish to pass.

The lake has been associated with the town of St. Moritz since the 19th century, when it became a destination for European aristocrats, and has hosted the White Turf horse races annually since 1907 on its frozen surface.

Lake St. Moritz serves as a venue for winter sports and social events, including polo matches played on ice every January or February and the sport of skijoring, which originated in this region around 1906.

The lake lies at 1,768 m, with St. Moritz's centre above the north shore and the mineral springs of St. Moritz-Bad at the western end. The St. Moritz railway station, terminus of the Rhaetian Railway's Albula line, opened in 1904 by the outflow. The southern shores and eastern end toward Stazerwald remain largely unbuilt. The ice usually clears only in May.

In 1988, Lake St. Moritz became the site of the first cricket tournament ever played on ice, and in 1990, a rental car driven by English cricketer David Gower broke through the ice and sank overnight.