Union secretary and Labor right-faction powerbroker Peter Malinauskas will fill the South Australian Upper House seat left vacant by disgraced MP Bernard Finnigan.

Mr Malinauskas was elected to the position by Labor's executive this afternoon.

The Shop, Distributive and Allied Employees Association (SDA) SA and Northern Territory branch secretary nominated earlier this week after receiving a request from Premier Jay Weatherill.

Mr Malinauskas said his time as a union secretary had left him with the necessary experience to make the step up to Parliament.

"For the last eight years I've worked really hard to represent the interests of working people," he said.

"Throughout that time I have developed good relationships with business and other parts of the community and I'd like to take those skills to the State Parliament and be able to make a contribution in what is a pretty important time in the state's history."

Mr Malinauskas said his job at the union would be filled by assistant Sonia Romeo, who will become the first female to be appointed branch secretary.

Finnigan quit politics last week after being found guilty of obtaining access to child pornography.