In a statement from Ms Ryan’s office released on Wednesday, she was seeking a position with more flexibility for her children.
“Serving my community as the first member for Euroa has been the honour of a lifetime,” she said.
Ms Ryan has served as Deputy Leader of the Nationals for the past eight years, and was the first woman in the party’s history to be elected to a leadership position at any level.
At the time of her election in 2014, she was the youngest member of Victorian Parliament.
“I want to thank our leader Peter Walsh and my colleagues for their friendship and unwavering support over that time,” she said.
State Member for Murray Plains and Victorian Nationals Party Leader Peter Walsh said Ms Ryan had been a driving force for positive change for regional Victorians.
“I know this is not a decision she’s come to lightly,” Mr Walsh said.
“Coming from a dairy farming family, Steph’s was one of many who struggled with the devastating environmental, social and economic impacts of the millennium drought.
“Steph has an unmatched passion, loyalty and dedication to her communities.”
He said Ms Ryan had always been driven by a strong sense of integrity and justice.
“Steph leaves this role, and her roles in the Shadow Cabinet, with a proud record of championing a better future for country people where it’s needed most,” Mr Walsh said.