Victorian Greens leader Greg Barber has resigned and confirmed he will leave state parliament, effective immediately.

The leader officially called time on his parliamentary career on Thursday, just as his party gears up for a crunch by-election in Labor's key inner-north seat of Northcote, where the Greens believe victory is within their grasp.

Greens leader Greg Barber says he's going to 'think about my future plans'. Credit:Wayne Taylor

Mr Barber, who holds a Northern Metropolitan seat in the state's upper house has led the party since 2010 and has also served as a local councillor and mayor of Yarra council.

Moreland councillor Samantha Ratnam​ will fill the casual upper house vacancy left by Mr Barber's departure.