The news comes as his caucus appears certain to grow to 23 members following the election and may unite with two moderate Democrats to better share power in the chamber.
Hewitt, who has served as minority leader since 2005, did have cancer surgery earlier this year.
In a statement, he said:
"I am now entering a new season in which I will turn over the leadership reins and focus my energies on the citizens of southeast Washington.
"This is the right time for me to step back and encourage a new model of leadership in the state Senate. I want to promote new Senate leadership from the center. Focusing on centrist policy rather than party labels is what will make the people of Washington a winner.
“Finally, my health challenges this past year have helped me focus on the important things of life. My wife Cory has been my biggest cheerleader as I have taken extra time devoted to public service of the state of Washington."
Hewitt's announcement means both political parties will begin the 2013 session with new leaders.
Earlier this month, Democrats chose Sen. Ed Murray, D-Seattle, as majority leader to succeed retiring Sen. Lisa Brown, D-Spokane.
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