NSW Greens upper house MP Justin Field is quitting the party because of internal division and "hyper-partisanship".
Mr Field plans to continue to sit as an independent in the Legislative Council and campaign against coal mining and for action on climate change and marine conservation.
He announced his decision online on Friday, citing concerns over the party's "hyper-partisanship and winner takes all approach", saying this had created "division and disloyalty" within the NSW Greens.
Mr Field said he wants to use the next four years in parliament to work on environmental issues rather than focus on internal reform.
He raised concerns about the March NSW election results which could put Shooters, Farmers and Fishers party and One Nation in the "box seat" in the upper house.
"An ideological opposition to working with any one side of politics is pointless when it manifestly fails to deliver outcomes and will ultimately empower the Shooters and One Nation," he said in his online statement on Friday.
The MP will join One Nation's Mark Latham on the crossbench.
The count for the Legislative Council is continuing for the final three spots, which could include Liberal Democrat and ex-senator David Leyonhjelm.
Mr Field, who entered parliament in August 2016 to fill a vacancy created after the death of Greens MP John Kaye, said some people would suggest he should resign from parliament altogether.
"Some will say I am only in this position because of the work of thousands of Greens members and I should, therefore, resign from parliament if I am no longer a Greens member," Mr Field said.
"Many who supported me in the job have left the party for many of the same reasons I have, and I intend to honour my commitment to them and the NSW community."
Mr Field said the rise of independents in Australia was partly driven by the division within political parties which he said seem "incapable" of working together.
NSW Greens co-convenor Rochelle Flood on Friday called on Mr Field to resign so his spot could be filled by a Greens member.
