As activists arrived in Harrogate, joint leaders Caroline Lucas and Jonathan Bartley sought to set out the party's role in modern British politics.
The party failed to add to its single MP in the general election and saw its vote share fall back from the 2015 result.
Labour's shift under Jeremy Corbyn's leadership has squeezed the space available on the left for the Greens, contributing to the party's poor performance in June.
Mr Bartley said that the Greens' approach to Brexit showed they offered an alternative to Labour and the Tories.
He said: "At autumn conference this year Greens will say the things others won't.
"So when we have a failing economy with stagnating wages and people working all hours to make ends meet, we'll say it's not enough to tinker with things.
"When there is an establishment consensus on leaving the single market, we'll say that's incompatible with ending austerity.
"We're at a turning point - and we need to be asking big questions about our future."
Ms Lucas, who retained her Brighton Pavilion seat and remains the party's sole MP at Westminster, said: "Only the Green Party recognises the scale of the challenge we face and the need for bold and fundamental reform so we can build a better future for all.
"We get to the heart of the issues - and ask who our economy is meant to serve.
"We stand up to the Government's callous policies towards migrants and refugees.
"And we are unafraid to politicise climate breakdown and set out the urgent policies we need to address it.
"We will always speak truth to power."
The party's conference runs until Tuesday.