The eastern boundary of Nashua is formed by the Merrimack River, and the city is drained by the Nashua River and Salmon Brook, tributaries of the Merrimack. The Nashua River roughly bisects the city.

Neighboring towns include:

• Litchfield, NH (NE)
• Hudson, NH (E)
• Tyngsboro, MA (SE)
• Merrimack, NH (N)
• Dunstable, MA (S)
• Amherst, NH (NW)
• Hollis, NH (W)
• Peperell, MA (SW)