Olney is a census-designated place and an unincorporated area in Montgomery County, Maryland, United States. It is located in the north central part of the county, ten miles (16 km) north of Washington, D.C.

Olney was largely agricultural until the 1960s, when growth of Washington, D.C.'s suburbs led to its conversion into a mostly residential area. It has a total population of 35,820 as of the 2020 United States census.

In 1763, Richard Brooke received a patent for a tract of land located in the Province of Maryland. Originally known as Mechanicsville, the village which became Olney was established in 1800. The area was mostly farmland, but it soon began attracting artisans. Early residents Sarah Brooke and Dr. Charles Farquhar were devotees of the English poet William Cowper, and named their home after the poet's hometown of Olney in England. The area was later named for their home, which still stands and is known as the Olney House. In the town's center was a blacksmith, William Kelley's wheelwright shop, Canby's pottery factory, and a Benedict Duley's store.

The Brooke family held the largest tracts of land in Olney, whose central village was at the intersection of the Rockville to Baltimore road, and the one which connected Washington with Westminster to the north. The Quaker community in Sandy Spring thrived just to Olney's east. The Sandy Spring Museum is a historical museum featuring educational programs and displays. St. John's Episcopal Church was established in 1842 and survives to this day.

Although not as involved in the Civil War as areas of Maryland to the west, Olney residents still experienced the tug-of-war between loyalty to the plantation economy of the South and to the Federal government in Maryland's midst. Both Union and Confederate forces made stops in Olney during the war. Union Generals George B. McClellan and Ambrose Burnside led soldiers through in the midst of the Maryland Campaign in 1862. During the Gettysburg Campaign in 1863, Confederate General J. E. B. Stuart marched between 10,000 and 20,000 troops north through the village and raided it of supplies, including horses and crops from surrounding farms in which they bivouacked.

Olney served as the original headquarters of the Emergency Management Institute, founded in 1951 as the Civil Defense Staff College (CDSC) to provide training for civil defense. While there, the college built "Rescue Street," a training center designed to resemble atomic bomb ruins. The CDSC was soon relocated to Battle Creek, Michigan due to security concerns of a potential attack on Washington, D.C. during the Cold War.

Olney has a total area of 13.0 square miles (34 km2), all land.

Olney's town center sits at the intersection of state route 97 (Georgia Avenue) and route 108 (Olney-Laytonsville Road). The town, larger than any other in the neighboring areas, lies south of Brookeville, west of Sandy Spring, east-northeast of Gaithersburg, north-northeast of Rockville, and north of Aspen Hill.

At the 2010 U.S. Census, there were 33,844 people, 11,606 households, and 9,447 families residing in the area. The population density was 2,603.4 inhabitants per square mile (1,005.2/km2). There were 11,879 housing units at an average density of 913.8 units per square mile (352.8 units/km2). The ethnic makeup of the area was 75.1% White, 11.9% African American, 12.0% Asian, 8.5% Hispanic or Latino, and 0.53% Native American. 3.1% of the population identifies with "Some Other Race."

There were 11,606 households, of which 81.4% were family households. Among all households:

The average household size was 2.91 and the average family size was 3.25.

28.9% of residents were under the age of 20, 25.8% from 20 to 44, 34.4% from 45 to 64, and 10.8% 65 years of age or older. The median age was 41.9 years. For every 100 women, there were 91.8 men.

According to a 2015 survey, the median household income was $126,762, and the median family income was $138,072. Men had a median income of $96,624 versus $68,709 for women. The per capita income for the area was $48,289. 2.8% of the population and 2.3% of families were below the poverty line.

Historic sites include Olney Ale House, Sandy Spring Museum, and Woodlawn Manor Living History Museum.

"Olney Days" is held annually each April, and included a parade and charity bike ride and walk.[citation needed]

Founded in 1938, the Olney Theatre is one of two state theaters in Maryland. It presents dramatic and musical live theatrical productions.[citation needed]

There is a branch of the Montgomery County Library.

Falling Green had been an active farm since 1764. A house and barn survive from the original farm. Falling Green house is a Georgian architecture house built in 1770 by a Quaker planter.

Public parks include Olney Manor Park (which features the Olney Swim Center), Southeast Olney Park, Longwood Park, Cherrywood Park, Bowie Mill Park, and the OBGC Park at Freeman Fields.

Olney is also home to a private country club and a golf driving range, and the Norbeck golf course.