Harris, of Arlington, is the longest serving Republican in the Texas Senate.
"I want to express my deepest gratitude to the people of North Texas and to my family for allowing me to serve," Harris said. "It has been the greatest honor of my life that my friends and neighbors have asked me to represent them alongside so many amazing men and women for almost three decades."
Harris has served in the Texas Senate since 1991. Before that, he served in the Texas House.
Last month, state Rep. Rodney Anderson, R-Grand Prairie, announced he planned to challenge Harris in next year's primary. At the time, Harris said he planned to run for another term.
Harris said he wants Victor Vandergriff to continue his work representing Senate District 9. Vandergriff serves as chairman of the Texas Department of Motor Vehicles. Last week, he stepped down as chair of the North Texas Tollway Authority.
"Senate District 9 needs strong leadership and I am encouraging Victor Vandergriff to step in and take over where I will leave off in the next legislative session," Harris said.
Vandergriff said in a statement that he intends to run for Harris’ seat as a Republican but is still discussing it with family, friends and community leaders. He plans to make a formal announcement on Oct. 11.
Harris' announcement adds to the unusually high amount of turnover coming to the 31-member Senate next year. Sens. Florence Shapiro of Plano, Steve Ogden of Bryan and Mike Jackson of La Porte, all Republicans, have also announced they don't plan to run for reelection.