Darryl Hickie, who represents the riding of Prince Albert Carlton, says it was an honour to serve for the last seven years.
Hickie is returning to active police duty with the Prince Albert police, where he was a constable before being elected on 2007.
Hickie announced last April that he wouldn't run again in the next provincial election, expected in 2016, and planned to return to the force.
He had come under scrutiny last spring because he trained to requalify as a police officer while still an MLA.
The Opposition NDP said at the time that some Prince Albert residents had qualms about a politician conducting police investigations involving his own constituents.
Premier Brad Wall said then he was OK with it, but that Hickie would have to quit as MLA before his term was up if wanted to return to active duty.
"It has been an honour to serve the people of Prince Albert Carlton as their MLA for the past seven years," Hickie said in a news release. "I look forward to continuing to serve the people of Prince Albert, just in a different capacity as a police officer."
Hickie's resignation is effective March 9. The Saskatchewan Party says because the resignation is so close to the next election expected this fall or next spring, the premier is not required to call a byelection, but can if he wants.
Hickie was minister for corrections, public safety and policing from 2007 to 2009 and most recently was appointed to lead an all-party committee on improving traffic safety.