Mr Justice Morgan ruled that the 19ha of land between Wyke Road and Chickerell Road should not have been approved as a green by Dorset County Council in 2001.
Betterment Properties launched court proceedings in 2005. Following a battle which has seen preliminary issues argued at the Court of Appeal, the judge ruled that it was "just" to order rectification of the Land Register to declassify the site.
The Open Spaces Society slammed the judgement. General secretary Kate Ashbrook said: "Not only does it threaten this much-loved open space, but it endangers the thousands of greens registered under the Commons Registration Act 1965."