Adelaide City councillor Robert Simms is set to swap local government for federal politics after being named by the Greens to fill a casual Senate vacancy in South Australia.

Mr Simms, 31, who has a law degree and has worked as a journalist, was announced as the senator-elect on Friday.

He says the creation of jobs in South Australia and the advancing of the renewable energy sector will be key priorities.

He also plans to offer strong support for same-sex marriage.

"As the first openly gay man to be elected to represent South Australia in the federal parliament, it will certainly be a key priority for me," Mr Simms said.

"(Prime Minister) Tony Abbott is doing all that he can to wreck progress on that social reform and I'll certainly be coming out swinging to advocate for the rights of all people in our country to get married."

Mr Simms will replace Penny Wright in the Senate after she announced her decision to quit politics because of a family illness.

He was selected from a field of six candidates voted on by the Greens' membership in South Australia, and was also the preferred choice among the party leadership.

His appointment to the Senate must be rubber-stamped by the South Australian parliament and once that occurs, he is expected to take up his seat in October.

But his stint in Canberra could be short-lived with his seat to be contested at the next federal election.

Fellow Greens senator Sarah Hanson-Young described Mr Simms as "competent and courageous" and pledged to work with him to "make sure you get over the line at the next election".

Greens leader Richard Di Natale said Mr Simms would be an exceptional asset to the party who brought with him a proven track record in representing communities.