Ayer is a census-designated place (CDP) and the primary settlement in the town of Ayer in Middlesex County, Massachusetts, United States.

[1] Grove Pond and Plow Shop Pond lie along the southern edge of the CDP, draining west toward the Nashua River, part of the Merrimack River watershed.

As of the census[5] of 2000, there were 2,960 people, 1,258 households, and 680 families residing in the CDP.

The racial makeup of the CDP was 88.31% White, 4.93% Black or African American, 0.24% Native American, 2.64% Asian, 0.14% Pacific Islander, 1.08% from other races, and 2.67% from two or more races.

38.6% of all households were made up of individuals, and 11.6% had someone living alone who was 65 years of age or older.