Stan Struthers was one of five senior ministers who left cabinet in November, saying they had lost confidence in Selinger's leadership. The resignations last fall brought an internal caucus rift into the open and prompted a leadership vote which Selinger barely survived.
But Struthers said the leadership vote, which Selinger won by 33 votes, didn't play a role in his decision.
"There is no question that I was disappointed at the result of the leadership contest," Struthers said Wednesday. "I had wished at the time there had been a different outcome but it is what it is.
"I think with me not running in the next election, that opens the door for someone to step forward and really represent the NDP well in Dauphin."
Struthers was Finance minister when the NDP increased the provincial sales tax in 2013, which saw the party slip badly in the polls. He was shuffled out of the portfolio and was Municipal Affairs minister when he resigned.
He said he discussed the decision with his family and they decided 21 years was enough.
"It's a tough job to begin with, being an MLA, no matter what side of the house you're sitting on," Struthers said. "I've earned every one of these grey hairs."
Struthers said he doesn't know what he'll do next but ruled out running for federal politics.
Selinger paid tribute to the Dauphin MLA in the legislature, commending him for 21 years of service, "the longest ever in the history of his constituency" before Struthers received a standing ovation from his colleagues.
Two other cabinet ministers who resigned alongside Struthers, Justice minister Andrew Swan and Finance minister Jennifer Howard, have already been nominated to run again.
Former cabinet ministers Erin Selby and Theresa Oswald, who challenged Selinger for the leadership, have not said whether they will run again.