Murray had already previously resigned his role as leader of the Senate Democrats following his mayoral election earlier this month.
In a letter sent a letter to Gov. Jay Inslee on Tuesday, Murray said that he wanted to ensure that there was sufficient notice for a replacement to be named before the start of the next legislative session in January.
Sen. David Frockt, D-Seattle has been the interim leader representing the caucus until Senate Democrats meet this week to elect a permanent leader.
Murray won the mayoral contest with 52 percent of the vote over current Mayor Mike McGinn.