Speaking at the Scottish Green Party conference in Inverness, the party's co-leader said independence is "a decision for the Scottish people".
His comments came after leading figures in the SNP floated the idea of foregoing a referendum if the UK Government refused to issue the Section 30 order needed to hold the vote - a move some have called "Plan B".
On Friday, SNP MSP Alex Neil said a majority for the nationalists in the 2021 election could provide the mandate for declaring independence without the agreement of the UK Government.
Speaking to the BBC, Mr Neil stressed that option could only be taken if Westminster refuses to issue the Section 30 order.
Mr Harvie said: "I think Scotland's future is as an independent country, part of the European family of nations.
"I think that's coming, maybe sooner than some of our critics want it.
"The fundamental thing about that, though, is that's a decision for the people of Scotland to make, on their time-scale.
"Independence can and only will be delivered in a democratic manner, that's the kind of Scotland I want to live in.
"I wouldn't want to live in a Scotland that had achieved independence by forcing it on people.
"It has to be a choice, it has to be, and I think will be, a democratic choice that the people of Scotland decide to make.
"They'll do it on their terms, not anyone else's."
Mr Harvie added that he believes there is already a mandate to hold a second independence referendum and the refusal of the UK Government to grant one could result in an even bigger majority of pro-independence parties returning to Holyrood in 2021.
Also at the conference, the Scottish Greens unveiled their top two candidates standing in Scotland's eight regions at the 2021 election.
Mr Harvie said: "I'd say [the Scottish people] have already elected a Parliament with a pro-independence majority and in the context of everything that's happened since Brexit, the mandate is there now to call for a referendum.
"If the UK Government refuses it, I think the people of Scotland will deliver another Parliament that re-asserts that mandate and woe betide any UK Government that continues to refuse to respect that democratic will."
First Minister Nicola Sturgeon has dismissed any possibility of declaring independence, saying it has to be done through democratic means.