Port districts are a special purpose district empowered to acquire, construct, maintain, operate, develop, and regulate harbor improvements; rail or motor transfer and terminal facilities; water transfer and terminal facilities; other commercial transportation, transfer, handling, storage, and terminal facilities; and industrial improvements.  A port district can levy property taxes, charge service fees for the use of its facilities, and issue bonds to finance its activities.  There are currently 75 public port districts in Washington.

Port districts are governed by a port commission.  A commission is comprised of three or five elected members who serve either four or six-year terms.  Commissioners are generally divided into districts.  If the port district is coextensive with a county with fewer than 500,000 people that has three county commissioner or council member districts, then the port commission's districts are the same as the county commissioner or councilmember districts.  If the port district is not coextensive with such a county, then the port district must be divided into three districts with approximately equal population.  Only a registered voter within a commissioner district can be a candidate for, or hold office as a commissioner for that district.  Additionally, only voters within that district may vote in the primary to nominate a candidate for commissioner from that district.  In the general election, however, all voters within the port district may vote to determine the district commissioner.

A commission may, with voter approval, increase from three to five members.  This change can be initiated through a resolution from the port commission, a petition from voters, or if the population of the county in which the port district resides reaches 500,000 people.  Once approved, the two new commissioners will be elected at the next election.  This will be the next general or special election if the change was initiated through a petition or resolution, or the next general election if the increase in commissioners was initiated because of the increase in the county's population. 

When a change to a five-member commission is approved, the port commission must divide the district into five districts by June 1 of the year in which the two new commissioners will be elected, unless the two new commissioners were approved to be elected at-large. 

A five-member port district with three members elected from commissioner districts and two members elected at-large members may be subsequently changed so that all members are elected from commissioner districts if voters approve of the change on a ballot measure initiated by a petition from the voters or a resolution of the port commission.  If voters approve the change, the port district must be divided into five commissioner districts within 120 days of the election.

A five-member port commission with three members elected from commissioner district and two members elected at-large that is approved by voters to transition to a district with all five commissioners elected from commissioners districts must divide into the five district prior to January 1 of the year in which the two additional commissioners are to be elected.  In the event that a port district is increasing from three to five members with five commissioner districts, the port district must be divided into the five commissioner districts by January 1 of the year in which the two additional commissioners will be elected.