Gaithersburg ( (listen) GAY-thərz-burg), officially the City of Gaithersburg, is a city in Montgomery County, Maryland, United States. At the time of the 2020 U.S. Census, Gaithersburg had a population of 69,657, making it the ninth-largest location in the state. Gaithersburg is located to the northwest of Washington, and is considered a suburb and a primary city within the Washington–Arlington–Alexandria, DC–VA–MD–WV Metropolitan Statistical Area.

Gaithersburg was incorporated as a town in 1878 and as a city in 1968. Gaithersburg is located east and west of Interstate 270. The eastern section includes the historic area of the town. Landmarks and buildings from that time can still be seen in many places but especially in the historic central business district of Gaithersburg called "Olde Towne".

The east side also includes Lakeforest Mall, City Hall, and the Montgomery County Fair grounds, and Bohrer Park (a well-known joint community recreation center and outdoor water park for kids and families). The west side of the city has many wealthier neighborhoods that were designed with smart growth techniques and embrace New Urbanism.

These include the Kentlands community, the Lakelands community, and the Washingtonian Center (better known as Rio), a shopping/business district. Consumers often come to this area during Black Friday and other shopping holidays for the deals and variety of huge brand name stores like Target and Dick's Sporting Goods, and smaller stores like Francesca's and Blue Mercury.

Two New Urbanism communities are under construction, including Watkins Mill Town Center (Casey East and West), and the massive "Science City". The state has a bus rapid transit line, Corridor Cities Transitway or "CCT", planned for the western portion of the city starting at Shady Grove Metro Station and connecting all the high density western Gaithersburg neighborhoods with a total of eight stops planned in the city. The National Institute of Standards and Technology (NIST) is headquartered in Gaithersburg directly west of I-270.

Other major employers in the city include IBM, Lockheed Martin Information Systems and Global Services business area headquarters, AstraZeneca, and the French multinational corporation, Sodexo. Gaithersburg is also the location of the garrison of the United States Army Reserve Legal Command. Gaithersburg is noted for its ethnic and economic diversity; it was ranked second for ethnic diversity among the 501 largest U.S.

cities, and first among smaller U.S. cities, by WalletHub in 2021.