Mr Harvie said a vote should be held during the Brexit transition phase, and warned that the SNP government was in “real danger” of missing an opportunity to secure independence.
His co-leader, Maggie Chapman, said a “wildcat” referendum staged unilaterally by the Scottish Parliament without the permission of Westminster would be a “very useful tool”.
“I think there’s a real danger in waiting too long on the hope that clarity, which may never come, is just round the corner,” Mr Harvie told a Scottish Sunday newspaper.
“To pass legislation for a referendum takes some time. If we wait too long, that won’t be doable in this current session of Parliament.”
The Green MSP added: “If this Brexit process is happening, I would like to see it [indyref2] … if we leave it even a few months longer we will have run out of time.”
Ms Chapman said that if an unauthorised referendum “came back in favour of independence, and the British government turned round and said, ‘No, you can’t have it’, I think that is another mobilising force”.
Scottish Conservative chief whip Maurice Golden said the Greens were “equally dangerous and selfish in their relentless pursuit of independence”.