The Washington State Patrol (WSP) is the state patrol agency for the U.S. state of Washington. Organized as the Washington State Highway Patrol in 1921, it was renamed and reconstituted in 1933. The agency is charged with the protection of the Governor of Washington and the grounds of the Washington State Capitol; security aboard the vessels and terminals of the Washington State Ferries; law enforcement on interstate and state highways in Washington; and providing specialized support to local law enforcement including laboratory forensic services, mobile field forces during periods of civil unrest or disaster, and tactical teams. The State Fire Marshal's Office, responsible for operation of the Washington State Fire Training Academy and for certain aspects of civil defense mobilization, is a component office of the Washington State Patrol, and the State Patrol is the managing agency of the Washington Fusion Center, which coordinates anti-terrorist and anti-organized crime activities within Washington.

State Patrol commissioned personnel, known as "troopers", have jurisdiction throughout Washington, with the exception of federal property and the territory of Indian nations.

The Washington State Highway Patrol was created by statute in 1921 to provide traffic enforcement on the state's principal motorways. In 1933 the force was reconstituted as the Washington State Patrol and organized as an armed, mobile police force that, in addition to traffic duties, could be rapidly deployed and concentrated in areas of the state undergoing public order emergencies. Six years later a Criminal Investigation Division was added and, in 1947, the WSP academy established in former U.S. Navy barracks in Shelton, Washington.

By the early 1960s the State Patrol had established a reputation as the state's most elite law enforcement agency, with more than 400 applicants annually applying for about 25 openings, and an annual turnover of about five percent. In 1965, the State Patrol was given sole jurisdiction of Interstate 5 through Seattle by the Seattle Police Department after previous collaboration.

In 1963 the Washington State Patrol began referring to its commissioned personnel as "troopers" instead of "patrolmen". The change was made to standardize practices in Washington with those of other states. In 1975 Cathy Swanson and Carolyn Pemberton, the first two female troopers, were commissioned. Twenty years later, in 1995, the first female chief of the State Patrol, Annette Sandberg, was appointed.

Eight troopers faced termination in a fake diploma scam discovered in 2009. Troopers who had earned a two-year degree were entitled to a 2% pay raise and those who had earned a bachelor's were entitled to a 4% pay raise. Eight troopers, who were identified during the course of a federal investigation into a diploma mill, were discovered to have submitted fake diplomas along with applications for a pay increase. A State Patrol spokesman reported that the agency intended to fire the troopers.

The State Patrol is administered by a chief who is appointed by the Governor of Washington to serve at his pleasure, by and with the consent of the state senate.

Specialized units of the State Patrol include SWAT, charged with providing tactical support in high-risk situations; the Rapid Deployment Forces, composed of five mobile field forces based in Tacoma, Bellevue, Spokane, Marysville, and Bremerton; the Motors Team, consisting of 42 motorcycle-deployed troopers operating on Interstate 5; the Honor Guard, providing ceremonial support during official funerals and other special events; and the Criminal Investigations Division and the Investigative Assistance Division, charged with investigating serious crimes or assisting local law enforcement in doing so, when requested.^{[citation needed]}