Helen Burnet, who represents the electorate of Clark in Tasmania's lower house, told reporters on Friday she would move the cross bench.
"This is not a decision that has been made in haste," she said.
"I have represented the Greens in local government and in state parliament for more than 20 years."
State party leader Rosalie Woodruff said Ms Burnet's decision to depart the Greens "has blindsided us".
"We acknowledge it will also be a big shock to the many people who put their faith in, and placed their vote with, Helen as a Green MP, only six months ago at the state election," Ms Woodruff said.
"There will be many members, supporters, volunteers and voters who feel let down today."
She said Ms Burnet had not indicated she was considering leaving the party until Friday and her exact motivations remained "unclear".
However, Ms Burnet hoped the move would allow her to amplify voices in her electorate on issues such as the divisive stadium build at Hobart's Macquarie Point, living costs, public transport and water quality.
"I've been frustrated about not being able to get my voice heard on those matters," she said.
Her departure from the Greens leaves the party with five MPs in Tasmania's state parliament.
"I feel this is the right decision for this point in time, and I know we have a very strong cross bench, and my scrutiny, and my criticism of the government will not change," Ms Burnet said.
"It's been a very difficult decision, but I feel that I must do this in order to have the best voice of my constituents on those important issues raised in parliament."
Ms Burnet has been a member of parliament for less than two years.
She previously served as deputy lord mayor of Hobart City Council and is a registered podiatrist who spent most of her career in the public health system.
Premier Jeremy Rockliff, Speaker Jacquie Petrusma and her parliamentary colleagues were told the news on Friday morning.
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