Long-serving South Australian Greens MP Mark Parnell will quit parliament early after previously announcing plans not to contest the state election in March next year.
Mr Parnell has served in the upper house for more than 15 years, but says he will retire from politics next week.
The Greens recently chose former senator Robert Simms to be the party's lead candidate at the state poll, giving him a strong chance of being elected to the Legislative Council.
Mr Simms will now be named as the party's nominee to fill the casual vacancy caused by Mr Parnell's decision.
"As I won't be re-contesting the next election, it makes sense to give Rob an opportunity to make his mark in state parliament and to become more familiar with key state issues before the election next year," Mr Parnell said on Tuesday.
"With his track record of service, I know that Rob will make a fantastic state MP.
"He has plenty of experience, he is passionate and enthusiastic and importantly, will be one of the youngest members of parliament."
Mr Simms, 36, served in the Senate for less than a year from 2015 after also filling a casual vacancy for the Greens but lost his seat at the 2016 election.
He currently sits on Adelaide City Council.
Mr Parnell is expected to present his farewell speech to parliament on Wednesday.