In the abrupt announcement posted on his website, the Redlands Republican called his nine years in the Legislature "some of the most fulfilling years of my life."
"It has been an honor to work alongside my colleagues in both the State Assembly and the State Senate - institutions that I hold in the highest regard," Emmerson said. "But it's time for me to move on."
Emmerson, who turned 68 Oct. 28, did not immediately return a message on the reason for his departure.
"In these past few months, my passion has waned and my constituents deserve a Senator with the level of commitment that I once had," he said of serving in the Democratic-controlled body.
"My health is perfect and I leave with the satisfaction that I worked hard and did my very best."
Emmerson was elected to the Assembly in 2004. He won a 2010 special election in Riverside County's former 37th Senate District to replace former state Sen. John Benoit after Benoit resigned to take a seat on the Riverside County Board of Supervisors.
Emmerson easily won re-election last November in the redrawn 23rd Senate District, which takes in swaths of Riverside and San Bernardino counties. He faced term limits in 2016.
Viewed as a moderate voice in the caucus, Emmerson was part of the group of five Republicans courted by Gov. Jerry Brown when the governor sought the Legislature's help to raise taxes. The career orthodontist spent several years working in the Legislature during the Reagan era before attending graduate school.
He later went on to head up a political action committee for the influential California Dental Association before his return to Sacramento. Emmerson currently serves as vice chairman of the budget committee as well as the committee on business and economic development.
Among the potential candidates to replace Emmerson in the Republican-heavy district are Assemblymen Mike Morrell, R-Rancho Cucamonga, and Tim Donnelly, R-Twin Peaks.
Donnelly, a conservative firebrand, this week kicked off his nascent challenge of Brown.
PHOTO: Sen. Bill Emmerson, R-Redlands, listens to discussion on the main budget bill as senators prepared to vote on the state budget in June 2011. The Sacramento Bee/Hector Amezcua