The 2004 Washington State Senate election was held on November 2, 2004, in which about half of the state's 49 legislative districts choose a state senator for a four-year term to the Washington State Senate. The other half of state senators are chosen in the next biennial election, so that about half of the senators, along with all the members of the Washington State House of Representatives, are elected every two years.
Twenty-four seats were up for their regular election this cycle, while four seats held special elections due to resignations by their incumbents, for a total of 28 seats on the ballot. Republicans held 15 of the seats and Democrats held the remaining 13. Democrats successfully flipped two seats in the general election.
The 2004 election utilized an open primary system. Primary results can be found here and general election results can be found here.