The Seattle Department of Transportation (SDOT) was formed by combining transportation planning from the former Strategic Planning Office (SPO) with the former Seattle Transportation Department (SeaTran) to bring a more comprehensive approach to transportation service delivery. A major element of SDOTs creation was the establishment of the Policy, Planning, and Major Projects division, which is charged with transportation system planning and providing increased control and influence over major projects under construction in Seattle.

SDOT is currently organized into eight divisions. The Director's Office, Human Resources, and Communications constitute the Executive management division. The Capital Projects and Roadway Structures division includes the Transportation Capital Improvement Program and operation and maintenance of the city's bridges and other structures. Street Use & Urban Forestry provides permitting for all work including planting, pruning, and removal of trees. Responsibility for long-range transportation planning, and developing transportation policy for the city of Seattle falls to the Policy & Planning division. Initiation of large projects and program development is lead by the Major Projects division. Financial oversight of the department, information systems and administrative support is directed by the Resource Management division. The Street Maintenance division has responsibility for street resurfacing, cleaning and general maintenance and Traffic Management is responsible for operation of the city's street system, and neighborhood and operational programs.

This department has had a long evolution beginning with the Department of Streets and Sewers which was responsible for planning, construction, repair, and cleaning of the City's streets, sidewalks, and sewers. City Council appointed Seattles first Street Commissioner in 1875. The position came under the jurisdiction of the Board of Public Works in 1890. The position of Superintendent of Streets, Sewers and Parks was established in 1896; authority over parks was removed in 1904. In 1936 the Engineering Department assumed the responsibilities of the Department of Streets and Sewers and the Traffic Department and the Department became the Maintenance Division of the Engineering Department.

An ordinance creating a Department of Transportation was passed in July 1971.The legislation provided for the appointment of the Director of Transportation, specified the duties of the Transit Advisory Board, provided for the Seattle Transit Commission, and transferred all remaining operational aspects to the newly organized department. The Seattle Transportation Department was created the traffic and transportation functions of the Engineering in 1997 when Department were consolidated. By June 2002, SDOT and SPO consolidated to form a newly organized department which absorbed responsibilities for maintenance and operation of streets, bridges, retaining walls and seawalls, and traffic control systems in the City. An ordinance passed in 2004 changed the departments name to Seattle Department of Transportation (SDOT).