As of the census of 2000, there were 86,605 people, 34,614 households, and 22,083 families residing in the city. The population density was 2,803.5 people per square mile (1,082.5/km2). There were 35,387 housing units at an average density of 1,145.5/sq mi (442.3/km2). The racial makeup of the city was 89.25% White, 2.01% African American, 0.32% Native American, 3.88% Asian, 0.03% Pacific Islander, 3.05% from other races, and 1.46% from two or more races. Hispanic or Latino of any race were 6.22% of the population.

There were 34,614 households out of which 31.6% had children under the age of 18 living with them, 49.3% were married couples living together, 10.4% had a female householder with no husband present, and 36.2% were non-families. 28.3% of all households were made up of individuals and 8.7% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 3.05.

In the city the population was spread out with 24.7% under the age of 18, 8.1% from 18 to 24, 33.5% from 25 to 44, 22.2% from 45 to 64, and 11.6% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 97.6 males. For every 100 females age 18 and over, there were 95.2 males.

The median income for a household in the city was $51,969, and the median income for a family was $61,102. Males had a median income of $43,893 versus $29,171 for females. The per capita income for the city was $25,209. About 5.0% of families and 6.8% of the population were below the poverty line, including 8.7% of those under age 18 and 6.4% of those age 65 or over.

Nashua's downtown is a regional commercial, entertainment, and dining destination. Recent plans have incorporated the Nashua River into the design of a pedestrian-friendly walkway.The large commercial districts located on Amherst Street (Route 101A) and Daniel Webster Highway are shopping havens, with the one at Daniel Webster Highway, including the Pheasant Lane Mall, attracting many people from Massachusetts taking advantage of the nonexistence of sales tax in New Hampshire.

The city is home to a number of technical firms, including Nashua Corporation, which was a leading producer of floppy disks through the early 1990s, making the Nashua name well-known in the world of personal computers. Defense contractor BAE Systems (formerly Sanders Associates) and computer firm Hewlett-Packard are the largest representatives of the high-tech industry prominent in the region. The Boston Air Route Traffic Control Center is located in Nashua.