The Friends of Sherwood Forest say the Â£5.3m centre, located on the edge of a country park, will devastate the area.
"It has all sorts of wildlife - insect life and newts," the charity's group spokesman Adrian Wilson said.
Nottinghamshire County Council said the new site would "balance the interests" of the community and visitors.
The current centre is in Sherwood Forest Country Park, on land designated as a site of special scientific interest but plans are to move it to Forest Corner, on grassland on the edge of the park.
The council has signed a contract with the Royal Society for the Protection of Birds (RSPB) to build and run a new one.
County council spokesman Derek Higton said: "The centre will be run by one of the world's leading conservation organisations - and that's why we partnered with them."
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"We are confident we can balance the interests of the community and the interests of the 350,000 people who visit Sherwood Forest and Major Oak every year."
Detailed plans have not yet been submitted.
Mr Wilson said: "It is priceless - if you were to drive up the road, on a Sunday lunchtime, you would see the cricketers out here all in their whites, people sat round watching the cricket match.
"You get that feeling straight way - this is England."
Shawn and Linda Geery, who walk their dog in the country park, said: "It is too near the edge of the forest, the parking problems will be horrendous."
Construction is scheduled to start in December 2016 with the opening expected by winter 2017.
A new visitors' centre proposed for Sherwood Forest will ruin "a priceless" tract of land and threaten wildlife, opponents say.