Lengby is a city in Polk County, Minnesota, United States. It is part of the 'Grand Forks-ND-MN Metropolitan Statistical Area'. The population was 86 at the 2010 census.
According to the United States Census Bureau, the city has a total area of 0.3 square miles (0.8 km²), of which, 0.2 square miles (0.6 km²) of it is land and 0.1 square miles (0.1 km²) of it (17.24%) is water.
As of the census of 2000, there were 79 people, 42 households, and 21 families residing in the city. The population density was 331.4 people per square mile (127.1/km²). There were 53 housing units at an average density of 222.3 per square mile (85.3/km²). The racial makeup of the city was 98.73% White and 1.27% Native American.