The 46-year-old Australian-born former journalist was declared the victor of a ballot of party members – beating off the challenge of closest rival Scots college lecturer Peter Cranie in the final round of a contest fought under the STV system of proportional representation.
The vacancy arose after the party's first and only MP, Caroline Lucas, stepped aside as head of her party.
Will Duckworth was elected as deputy leader, the party announced after all votes were counted, winning a close battle between the two male candidates eligible for the number two role.
Ms Bennett will give the keynote speech at the party's autumn conference, taking place in Bristol from Friday.
She said Green Party policies were "the only viable way forward for British people, for the world".
Ms Bennett added: "We need to not have the economically illiterate cuts we are seeing now; what we need to have is investment in the future. Investment in homes, investment in jobs, investment in energy conservation and renewable energy and public transport."
And the economy needed to be "relocalised", returning manufacturing to the UK and supporting farmers in providing food, she said.