Arlington is a city in northern Snohomish County, Washington, United States, part of the Seattle metropolitan area. The city lies on the Stillaguamish River in the western foothills of the Cascade Range, adjacent to the city of Marysville. It is approximately 10 miles (16 km) north of Everett, the county seat, and 40 miles (64 km) north of Seattle, the state's largest city. As of the 2020 U.S. census, Arlington had a population of 19,868; its estimated population is 21,959 as of 2024.

The city lies in the traditional territory of the Stillaguamish people, an indigenous Coast Salish group. Arlington was established in the 1880s by settlers and the area was platted as two towns, Arlington and Haller City. Haller City was absorbed by the larger Arlington, which was incorporated as a city in 1903. During the Great Depression of the 1930s, the Arlington area was the site of major projects undertaken for employment under the direction of federal relief agencies, including construction of a municipal airport that would serve as a naval air station during World War II. Arlington began suburbanizing in the 1980s, growing by more than 450 percent by 2000 and annexing the unincorporated area of Smokey Point to the southwest.

The economy of the Arlington area historically relied on timber and agriculture. In the early 21st century, it has transitioned to a service economy, with some aviation industry jobs near the municipal airport. The city is governed by a mayor–council government, electing a mayor and seven city councilmembers. The municipal government maintains the city's parks system and water and wastewater utilities. Other services, including public utilities, public transportation, and schools, are contracted to regional or county-level agencies and companies.

The indigenous Coast Salish peoples have inhabited the Puget Sound region since the retreat of the Vashon Glacier approximately 12,000 years ago. Several archeological sites along the Stillaguamish River contain artifacts that are dated to the Olcott Phase, approximately 9,000 years before present. The traditional territory of the Stillaguamish people includes modern-day Arlington and much of the Stillaguamish River basin; they had at least ten known villages along the river and its tributaries, including two in the Arlington area. Among them was sq'ʷuʔalqʷuʔ (meaning "confluence"; variously anglicized as Skabalko or Skabalco), a settlement with winter longhouses and 200–300 people at the confluence of the two forks of the Stillaguamish River. Other Coast Salish peoples who were intermarried with the Stillaguamish would travel to this village in the summer to follow fish runs. The neighborhood of Kent Prairie (Lushootseed: xʷbaqʷab) was once a prairie where the Stillaguamish, Sauk, and Snohomish would gather wild crops. In modern Lushootseed, Arlington and the surrounding area is named stiqayuʔ, meaning "wolf".

American exploration of the area began in 1851, when prospector Samuel Hancock was led by Indian guides on a canoe up the Stillaguamish River. The area was opened to logging after the signing of the Treaty of Point Elliott in 1855 between the federal government and various Puget Sound peoples. The Stillaguamish, one of the signatory tribes, were ordered to travel to the Tulalip Indian Reservation, but many members refused and remained in their ancestral lands. After attempts in the early 20th century, the tribe were granted federal recognition in 1976 and a reservation in 2014.

In 1856, the U.S. Army built a military road connecting Fort Steilacoom to Fort Bellingham, crossing the Stillaguamish River near the confluence. In the 1880s, wagon roads were constructed to this area from the towns of Marysville to the south and Silvana to the west, bringing entrepreneurs to the logging camps, informally named "The Forks". The area's first store was opened in 1888 by Nels K. Tvete and Nils C. Johnson, and was followed by a hotel with lodging and meals for loggers.

Two settlements were established on the south side of the confluence in anticipation of the Seattle, Lake Shore and Eastern Railway building a track through the area. G. Morris Haller, son of Colonel Granville O. Haller, founded a settlement on the banks of the Stillaguamish River in 1883, naming it "Haller City".

The Seattle, Lake Shore & Eastern Railroad chose to build its depot on higher ground to the south of Haller City, leading contractors Earl & McLeod to establish a new town at the depot on March 15, 1890. The new town was named "Arlington" after Lord Henry Arlington, member of the cabinet of King Charles II of England. Arlington and Haller City were platted within a month of each other in 1890, quickly developing a rivalry that would continue for several years.