Victorian secretary of the Australian Workers Union, Cesar Melhem, will become an state upper house MP.
Mr Melhem will represent the Western Metropolitan Region in the state legislative council, filling the vacancy left by Martin Pakula.
Mr Pakula is moving to the lower house after winning the Lyndhurst by-election last month, following sitting member Tim Holding's resignation.
Mr Melhem said although he has decided to leave the union, he will still serve Victorians in his new role.
"I know first-hand the challenges that workers and their employers are facing at this particular time in our state's history," he said.
"I am humbled by this opportunity to take my contribution to the next step as a member of the Victorian Parliament."
Mr Melhem was endorsed by the ALP's national executive on Friday.
Long-serving AWU official, and member of the union's national executive, Ben Davis, is expected to be formally endorsed as the new AWU Victorian secretary later this week.