Glenstone is a private contemporary art museum in Potomac, Maryland, near Washington, D.C., founded in 2006 by American billionaire Mitchell Rales and his wife, Emily Wei Rales. The museum's exhibitions are drawn from a collection of about 1,300 works from post-World War II artists around the world. It is the largest private contemporary art museum in the United States, holding more than $4.6 billion in net assets, and is noted for its setting in a broad natural landscape.

Glenstone's original building was designed by Charles Gwathmey, with it being expanded several times on its 230-acre (93 ha) campus. Its most significant expansion was finished in the late 2010s, with outdoor sculpture installations, landscaping, a new complex designed by Thomas Phifer, and an environmental center being added. Glenstone has been compared to other private museums, such as the Frick Collection and The Phillips Collection. The museum is free to the public, with over 100,000 visitors in 2022.

In 1986, billionaire American businessman Mitchell Rales purchased the property in Potomac, Maryland, to build a home. Starting in 1990, Rales began collecting art for that home. Following a near-death accident on a helicopter trip in Russia, Rales decided to take on a philanthropic project, which became the establishment of a private contemporary art museum. Built on land that was formerly a fox hunting club, Glenstone is named for the nearby Glen Road, and because of stone quarries located in the vicinity. Located 15 miles (24 km) from downtown Washington, D.C., the museum's initial 30,000-square-foot (2,800 m²) Modernist limestone gallery opened in 2006 and admitted visitors two days a week. In its first seven years, the museum admitted only 10,000 visitors.

In 2015, Glenstone was one of several private museums questioned by the US Senate Finance Committee over its nonprofit tax status. After reporting from The New York Times had questioned the validity of nonprofit tax status for institutions like Glenstone, which, at the time, welcomed very few visitors, the committee sought to investigate whether high-value individuals and families were using private museums as a form of tax shelter. Committee Chairman Senator Orrin Hatch said in a letter to Glenstone and other institutions (including the Rubell Museum in Miami and The Broad in Los Angeles) that, "Some private foundations are operating museums that offer minimal benefit to the public while enabling donors to reap substantial tax advantages."

Though several smaller expansions took place in the years after the museum's opening, the largest expansion was announced in 2013 and was completed in 2018, opening to the public on October 4, 2018. With a cost of approximately $219 million, the expansion increased the size of the museum's gallery space by a factor of five, increasing the property's size by 130 acres (53 ha), and included substantial landscaping changes. With the expansion, Glenstone became the largest private contemporary art museum in the United States. In 2019, the expansion was named as a "Museum Opening of the Year" by Apollo.

With the expansion, Glenstone opened to the public, with free tickets available online. In the year following the expansion, Glenstone admitted nearly 100,000 visitors.

After 2018, Glenstone continued to acquire adjacent properties, including lots containing residential homes. Speaking on Public Radio Tulsa's Museum Confidential program in 2019, curator Emily Wei Rales said that plans do not include substantial expansion, and will likely be limited to "one or two smaller buildings to house artworks, maybe in the woods, maybe on an adjoining property". She also mentioned potential plans to build a conservation lab on the campus.

To encourage the usage of public transportation, Glenstone successfully lobbied Montgomery County to add a bus stop near its campus. The museum admits visitors without tickets who arrive on public buses.