Lapwai is part of the Lewiston, ID-WA Metropolitan Statistical Area.
According to the United States Census Bureau, the city has a total area of 0.8 square miles (2.0 km²), of which, 0.8 square miles (2.0 km²) of it is land and 1.28% is water.
There were 339 households out of which 48.1% had children under the age of 18 living with them, 40.1% were married couples living together, 27.1% had a female householder with no husband present, and 20.1% were non-families. 17.1% of all households were made up of individuals and 6.8% had someone living alone who was 65 years of age or older. The average household size was 3.35 and the average family size was 3.68.
In the city the population was spread out with 39.8% under the age of 18, 9.3% from 18 to 24, 26.6% from 25 to 44, 17.7% from 45 to 64, and 6.5% who were 65 years of age or older. The median age was 26 years. For every 100 females there were 92.2 males. For every 100 females age 18 and over, there were 90.8 males.
The median income for a household in the city was $26,800, and the median income for a family was $30,417. Males had a median income of $31,382 versus $22,109 for females. The per capita income for the city was $10,159. About 20.5% of families and 23.7% of the population were below the poverty line, including 30.5% of those under age 18 and 10.2% of those age 65 or over.